To complete the network, relays are required on ATProto and apparently could be expensive to host, so right now, it appears the only relay is hosted by BlueSky the company. Which makes things slightly centralized.
A number I've seen quoted multiple times now is ~$150 per month to host a relay (Source). Which explains why Whitewind, Smokesignal and Frontpage don't host one, they are mostly still small projects by individual talented devs, but imo if that number is true, it really doesn't seem too outlandish that someone might go for it.
I don't think that's necessarily true. As fas as I know there are no plans to inject ads, they are planning to create a marketplace for custom feeds (think "premium" feeds) and labelers and such where they would take a cut. You would obviously still be able to purchase access to them from other servers. But this goal seems kinda lofty, not sure if it can be viable.