Allrighty, the real reason IMO the IC's CG isn't banned is very simple:
IT IS PREVENTABLE
simply hitting Nana once, then grabbing Popo means that you have separated them. now if one grabs you, the other should be in no position to re-grab to start shenanigans. From there, they need to either release you, or throw you as they have really nothing better to do until their partner returns.
Simply racking an extra few % insures that one of them goes a different distance, use that to your advantage if you get a hit in. It's even better in the Air as nana is a ******, and usually doesn't DI well if the IC player focuses on popo for the moment.
(another thing to capitalize: Not only is Nana dumb, but the IC player needs to focus on two characters getting together if you separate, and not necessarily on grabbing you to start shenanigans)
Simply playing smart against the ICs and abusing the weakness of separating them (eliminates their recovery, Grab shenanigans, and makes IC player lose a bit of focus as they try to get Nana/popo back together) should allow you to beat them, and prevent a CG.
Remember, you only get CG'ed by them if you let them do it to you.
Now to go off an a bit of a tangent here but, if any CG should be looked at for banning...it's
DDD's.
When you think about it, it doesn't necessarily follow the criteria for ban in the traditional sense we use here on SWF, seeing as DDD doesn't
technically dominate with it alone, but he does however control the metagame with that one move.
-DDD's CG is the basis for many stages being banned (other chars have CG's that work on the stages, but they are all either preventable, or escapable after x%)
-You honestly can't play "don't get grabbed" with a DDD, their grab is too fast and too far-reaching to simply not get caught by. Hell, even our best players cannot avoid this, ad they're the guys who supposedly "don't get grabbed".
-DDD's dthrow alone shuts down characters. The poster-boy for this of course is Donkey Kong, DK can pretty much go toe to toe with the entire cast, but gets stopped cold and thus made essentially unviable due to one character's Dthrow. I don't care what you say about other chars, or other MU's, that is ********. If his Dthow CG were at the least limited, about 7 characters would become viable (especially DK) and several stages would come out of banned mode, or at least into CP.
-DDD's metagame essentially revolves around the CG. He does have other col things to offer like his minions (Laser one can lead to shenanigans) and Bair walls, but what keeps him so high is that Grab of his. If you take a look at his MU's, the only characters he loses to either cannot be CGed (IC's, Pikachu), can simply avoid the CG by preventing DDD from getting close (Falco, MK) or have an even gayer grab (olimar). That alone sounds fishy, if his only bad MUs are the ones where he doesn't have something to exploit, what does that say about his Good ones?
TL;DR
IC's CG isn't bannable because it is not only avoidable, but preventable with smart playing.
DDD's CG is the real one to be worried about, he scews the metagame even more than Meta
Game Knight just by having a Dthrow.