I don't think it's about poor kids asking for less or that they shouldn't ask for anything fun.
I think it's about the idea that someone, not related to you like your parents or grandparents, buying you a gift and it really shouldn't be a ridiculously expensive one.
When my parents or brothers ask my kids what they want for Christmas, I know they are going to ask for something simple like toys, books or clothes. (something still "fun" to them but not expensive)
When my DH and I ask them what they want for Christmas, the list is more extravagant. (Xbox, games, laptop, etc)
I would find it in bad taste to pick up a card from the Angel Tree and find XBOX on the list. I'd put that one back and look for another one. I wouldn't mind finding a "want" on the list, within reason. I just don't think an XBOX, for example, is reasonable. After all, the games cost a pretty penny.