It's not like that. They buy mcnichol.com on your behalf, when you request it. They're not registering it in advance and then blackmailing you to rent it through them. Domain is yours, you can move it elsewhere (they explicitly mention that).
It's same as any other domain registrar, except that have small markup (couple of dolars per year) but they provide additional service, i.e. customer doesn't have to deal with DNS.
They buy the domain their customer _wants_ them to buy. That customer is free to transfer it out if they want to. What are you going on about?
They buy 900 domains. They hold 900 domains.
Anyone who wants that domain cannot use it but must rent through them (whom they rent through someone else)
This is textbook rent seeking behavior.