The police are muscle that applies violence to advance the interests of the state. They only protect the wealthy insofar as that serves the interest of the state. The interests of the wealthy are generally fairly aligned with the state for various reasons. There is a subtle but important distinction between that and "serving the rich".

> They only protect the wealthy insofar as that serves the interest of the state.

In a capitalist society, that's tautological. The interests of the wealthy are the interests of the state.