Doesn’t discrimination need to be allowed so the site can ban people for things without breaking their own rules? // Banning in one way or annother is kind of the definition of discrimination.

No. It's basically two different meanings associated with the same word:

• "to discriminate" as "to sort out", "to make a distinction" - necessary to ban users
• "to discriminate" as "to treat users differently based on social or individual prejudice" - what people shouldn't be doing

The first meaning is mostly used formally. The second one is kind of "default" when people talk about discrimination.