You can't connect to an "unpaid" server with the android client (except in the first 30 days, as a trial). So every server would be a "paid" server.
Sindre wasn't making enough money to support his development efforts so he added a second revenue stream to the process... either the ads or pay to remove the ads. This is separate and in addition to the money that you pay to license the server so it can accept API connections past 30 days.
Glenn