They filtered all of the punctuation because they apparently thought that you could use them to bypass the filter. For example, you could use "/" in the place of "L" so the word you were saying wouldn't get filtered. It isn't the actual smilies, just the punctuation used to make them (aka the parentheses and the colon/semicolon).
I do think they go way overboard. Most of the people using the chat don't even have the faintest clue why "muffin" or "pudding" are censored. They also censor the letter "B" (found that out the hard way trying to tell a friend to do a B office...
).
I agree that whether the kid is chatting with inappropriate people should be in the hands of the parents, not Disney. All the parents have to do is restrict their child's secret friend access and they'd never have to worry.
They're equally as paranoid about the type-a-name thing. But that's a whole other ordeal...