"Racism" can mean interpersonal ("I don't like black people") or it can mean systemic ("society disadvantages black people"), and lots of people in social studies focus on the latter. Because there's not really any systemic disadvantage to being white, that focus on the latter effectively defines racism against white people as non-existent (there's not systemic racism against white people, so there's not "racism" against white people). Given that, "racism against white people" is definitionally not "racism", hence "reverse racism".

Of course there are multiple steps in there which can be criticized, but within the set of terms they've defined, it makes the language less ambiguous. It's like how astronomy often defines "metals" as "anything bigger than helium". The terms have a useful meaning in the proper context.