It actually makes sense that way. If someone is your advocate, customer, sponsor, etc. it generally means you are in a subservient role, if only to the extent that you need something like money from them.
Think of Victorian England, when having a trade was a bad thing. Real gentlemen lived off their land and their investments. While the word itself is much older, I think a lot of the modern connotations come from that mode of thinking. Lord So-and-so might patronize your shop, but you have no way to patronize him because he has nothing to sell to the likes of you!