Why does Ponyboy hope that Bob's parents hate him?

When Ponyboy states his preference for Bob's parents' hatred rather than their pity, he is stating his preference for equality and dignity over condescending sympathy. If someone feels pity for someone else, he or she is acknowledging that the other person is lacking something or suffering from a misfortune of some kind, often in a patronizing way. Ponyboy feels resentment toward anyone who feels pity for him and for the other greasers because he is a proud person who believes that he and his fellow greasers are coping just fine with what they have and with what they try to provide one another.
If someone feels hatred, that means that both parties are on equal footing; if someone feels pity, that means that one party is positioned higher than the other. No matter the reason, Ponyboy has the strength of spirit and the pride in himself as a human being to resent the suggestion that the greasers are deserving of a lower status of any kind.

Ponyboy doesn't necessarily want Bob's parents to hate him. What Ponyboy says in chapter 11 is that he would rather have their hate than their pity.

I hoped they hated us, that they weren't full of that pity-the-victims-of-environment junk the social workers kept handing Curly Shepard every time he got sent off to reform school. I'd rather have anybody's hate than their pity.

The reason that Ponyboy would rather have their hate than pity is that he believes pity will cause people to look down on him and his fellow Greasers. Pony is okay with being poor and coming from the rougher side of town. He accepts this, and he never once lets those things make him feel like a less worthy person. Pity makes him feel this way. If Bob's parents pity him, they are essentially looking down on him. Ponyboy is not okay with that. If Bob's parents (or other Socs) hate him, that means they consider him a worthy rival and opponent. They would not look down on him. They might even fear him. That makes Ponyboy an equal to them in a lot of ways. Pity implies that there is something wrong with Ponyboy and his Greaser friends, and Ponyboy doesn't see anything wrong with the way that he lives.