when it comes to double posting, infracting someone who does it is basically just an attempt to keep the forum clean/organized. you're absolutely right in saying that it isn't a big deal, especially when compared to some of the other problems people can cause on a forum that merit moderation, but in the same hand, it isn't difficult to edit your post instead of double posting and keeping everything you say in one post keeps everything more organized. you wont be banned by a double posting infraction unless you've done it six times or you've broken other rules, so its more of a measure to keep things neat.
on going off topic, it can become a problem when people are posting in a thread about one subject when there is a whole forum dedicated to that subject somewhere else in the forums. not only is it more organized to ask your falcon-sheik matchup question in the falcon board, but you will understandably get better answers there, too. it can also become a problem when it leads to talk about the other subjects you bring up: sex, violence, and drugs.
bottom line in a legal sense, if we ignore the information people post on the website, we are responsible for it on the same level as if we allow it. having the rules doesnt cover us if we fail to enforce them properly, and it shouldn't, as then the rules would be meaningless. Smashboards is and always has been a community that children are encouraged to join, even if Brawl received a Teen rating from the ESRB for "cartoon violence" and "crude humor." We have to hold ourselves to the standards of a website fit for children, even if teenagers and matured adolescents end up making up the majority of our population. many parents frown upon their children coming across sex discussion, because it can be inappropriate for young people, especially when older posters get involved who aren't considerate of naivete or innocence. it can also lead to an interest in pornography, which despite anyone's moral opinions, is illegal to access until you are older than 18. Similarly, drug related discussion can lead to an interest in drug use, which despite your or my moral or intellectual view, is illegal. All of this is aside from and not to mention moral opinion, but when that is included, we have to stand by the fact that we as a community want to encourage participation. in that sense, it is only natural to disallow certain subjects and get more involvement than it is to allow certain subjects and shun possible users who could benefit from our website. in the end, it is better for the smash community to have more players, especially young ones who could sustain interest in the game for a long time and keep the community alive. young blood, so to speak. and the younger members would need to convince their parents that our community is safe and they should be allowed to play games with people their parents may consider "strangers," and they should be allowed to sleep-over at a tournament venue out of state, and that they would be safe doing so. this sense of safety is incredibly, majorly true for our community, but talk about violence (and sex, and drugs) could dissuade parents from allowing their kids to become the next mango or jman.
on your last subject, mentioning that people would only abuse this sort of discussion in the same manner that they would offline in a real discussion, i have to object to that statement. people act differently online than they do in real life. more than many of us would like to admit are guilty of that. trolling is an issue that makes everything much harder to manage, people can act far more immature or belligerent behind a computer screen than they can in real life. after all, who is going to stop them from offending someone else if they are alone, miles and miles away? the answer has to be the site itself, the moderation team and the administration, because the person they are enraging or offending can't be expected to respond in any sort of personal or physical manner without fueling the flaming topic.
people might or might not talk more about drugs on a forum than they would in real life, people might or might not talk more about violence than they would in real life. Most importantly, however, is that they are talking with a different audience on the forum than they would in real life - they aren't just talking with their teenaged or of-age friends, but anyone or any child who reads what they post. there are some things you just wouldnt say in front of a kid, and you cannot realistically pick and choose who reads what you post on a public forum.
on the fairly different topic of mods treating non-mods differently or worse than they would each other, i can only give two pieces of advice. the first, honestly and without intent to offend, is that you step back from the situation and try to judge whether or not they are really treating you differently or if you are just upset because of another reason. if that isn't the case, then you're encouraged to take the problem to Forum Disputes -- please, while this might seem counter-intuitive (bringing problems about mean/hateful/ignorant moderators TO the moderators?), you should know that Moderators cannot read Forum Disputes and neither can Super Moderators, only the administrators can. The people you would be dealing with are McFox, Amorasaki, Azua (aka Queen Killjoy), Scav, or MLG_JV (aka Jv3x3). Each of these admins are older than the majority of the moderation team, consistently more mature (which is saying something), and unafraid to de-mod someone or at the least put someone on suspension for being wrong or abusing their power.
i hope this helps, and i promote this kind of open discussion as long as it remains reasonable. if you have any questions, feel free.