Link also inexplicably fights like his OoT self in Melee. Funnily enough, he's the same person in 64 and Melee, but fights differently in both. Huh.
That still doesn't explain why Zelda in Melee and Brawl fight very similar when their both based off different Zeldas from tLoZ games. Yes, Link was pretty much based off the same Link in both the original Smash and Melee, but he wasn't the only one who had moves changed. A few others like Mario and Captain Falcon had moves changed too. However, I'm not really sure if it's fair to compare Link. The different Links seem to share a lot more in common with each other then the different Zeldas. All Links have at least some sort of bombs, arrows, and boomerang. The Zeldas on the other hand, seem to have a much bigger range on what they do, if anything besides getting captured (at least in the older tLoZ games).
Those two Zelda reps, Link and Zelda, are a strange subject. They keep being reborn, in a way. They LOOK similar... so it's not far-fetched for them to act similarly.
However, they ARE different people if you care about the established timeline.
My point was how people stressed so much about OoT Zelda and TP Zelda being as if they were two totally different people (well in a way, they are, but at the same time, not), yet I don't see people complaining about the TP based Zelda in Brawl fighting like the OoT based Zelda from Melee, when suchs spells never existed in TP. They say how those simple spells could have been passed on, but say a transformation couldn't have.
However, how much of this story connection will be taken into factor in Brawl, a non-canon game to every series in it? Now that we seem to know for sure Brawl is gonna have a storyline, this seems to be way up in the air with how this will all be done. There's still the "their trophies" factor, which could mean they could have various degrees of how close they match the originals, some closer then others. In other words, do we even have enough proof for the debate to go anyway that's not in circles?
OoT Zelda herself never fought much besides holding Ganon in place with a beam of light. Her moveset in Melee is a generic "Zelda" moveset not specific to her character in OoT, besides Sheik.
I don't know if this really counts for much at all, but her B moves were based off of spells from OoT, all different then how they were used by Link. I'm not really so sure if that could be considered generic or not seen I don't recall those spells in other Zelda games. Of course, this doesn't change the fact that she did transform into Sheik within the game, unlike the OoT spells given to her in Melee. That's why I'm not sure if this bit I'm saying counts for much at all.
One could argue that the Goddess spells used by Link in OoT are ties to OoT, and I agree. But there's no reason why TP Zelda would not have access to them, as these spells are probably passed on throughout the royal family, and the Goddesses, being immortal, are consistent throughout the whole series.
Sheik, on the other hand, is VERY specific to Ocarina of Time Zelda, and this Zelda we have in Brawl, is NOT her. It's probably the one from TP, or simply a generic one, but she has no more ties to Sheik than she does to Tetra.
So you say those spells could easily be pased on right? Who's to say transformation spells can't be either? Before "what other Zeldas besides OoT and TWW Zelda transformed?" comes up, well, what other Zeldas used the OoT spells? If the Zelda in Brawl was given spells that the other Zeldas didn't have, then why couldn't she also be given a transformation too? How come they have to limit themselves to just one Zelda? Just because the Zelda in Brawl is designed to look like a particular Zelda doesn't mean they have to limit themselves to that particular Zelda's canon and moves displayed in her own game.
Now on another note...
I have a feeling the developers won't overlook the massive lack of balance between Zelda and Sheik if she returned again. Simply making Sheik her own character slot alone doesn't change the fact that those two are still very unbalanced from each other and that Zelda could still be overlooked. Heck, I could see Zelda being overshadowed more if she could transform into a much more broken character. I expect Zelda will be buffed up and Sheik nurfed. After all, did they not nurf Sheik (and a few others) on the PAL verison of Melee? I think their aware of the poor character balance issues between the two. I'll be hard pressed to believe they wouldn't change the returning characters for Brawl.