The MAIN issue with all this that Sealy was addressing is that you can't have 'premium players', or anything like that. Which, yes, you can easily paint as evil - but they're what run a lot of servers. I know a pretty large number at least have things like limits on how much land you can apply grief-protection plugins to, or how much you can chunk-load at once. You know, things which - in greater amounts - put greater strain on the server. You can play fine without them, but having them is nice for more dedicated players. This isn't "10 bucks to access the nether", it's "10 bucks to use up enough extra memory on our server that we need your 10 bucks".
Most servers will be fine, yeah. But minecraft servers benefit from a large player base. Thus, they benefit from allowing people to play for free, because it enhances the experience of the paying players. This removes their ability to actually encourage paying in any gameplay-significant way, even IF the core experience is left alone. Will this shut down tons of servers and spell the end of minecraft or something like that? No, of course not. But it is banning something that a lot of very good servers need to cover their operating costs at the standard of service they currently provide.