The rich could choose to be less wealthy if they wanted. The poor are (overall, collectively) in a situation that they cannot escape. This means that being rich is a choice while living in poverty is an inescapable identity.
So when the poor "discriminate" against the rich, big deal. If you don't want people to hate you, give up your privilege. If you're a cop and you think you're the victim of hate, quit your job. Same thing.
But when the rich discriminate against the poor, their oppressees, they are making an attack on an identity like any other. This is why the poor can ethically hate the rich, but not the other way.