if they're distributing under gpl, that means they 1) have to make the source available, 2) the user maintains the ability to redistribute.
So provided that the user makes their own apt repo, they should be able to distribute it to as many machines as they see fit. That's essentially how centos operated. I'm sure there's some exceptions, around assets that ubuntu may own or any trademarks. But I shouldn't be learning this from an HN link to a twitter post of a screenshot that references ubuntu.com/pro. So I have the sense they haven't done much if any advertising for it? Maybe I've just been oblivious?
It is somewhat concerning that they're kinda holding security packages hostage...but I don't think it's unreasonable for ubuntu to want corporations to chip in. Gotta keep the operation running somehow, and bills don't just pay themselves. I don't know what else they can offer to make anyone actually shell out some cash.