Why ban alignments?

[Fax Celestis]

If the rogue's lawful, then he shouldn't be doing it anyway. If he's chaotic evil, then he's supposed to be doing that. And that's why you ban that alignment.

And if he's chaotic good, stealing from bad people to provide to the poor?

Because the DM wants to play a certain style of game that's incompatible with a party with an evil player. Players don't like in when other players screw over their characters. DMs don't like it when players screw over their world.

You still haven't demonstrated how playing, say, Faust is "screwing over their world".

Alignment is descriptive, not defining.