Yep, you're right. Personally I doubt that whoever designed this was so twisted they intentionally meant this as an insult to the victims of 9/11. But it is certainly in highly questionable taste to include a memorial feature that can be interpreted as an Islamic symbol - even if that crescent would actually look pretty impressive.
Do we have to ban crescents? No, of course not. But including them in 9/11 memorials seems dumb and insensitive. Personally, I'd leave it up to the families to decide.