The difference is that this is just a website that offers subdomains and not a TLD. co.uk and com.au operate very differently through a network of registrars and such. The same company that put restrictions on the .uk TLD also manages the co.uk domain in a similar fashion to other TLDs, for example.
You won't see co.uk ever expire because it's owned and manages by the same company that manages the TLD, but it's perfectly possible for eu.org to mess up their renewal and disappear from the internet. There's no direct connection between .org and the company managing these subdomains.
Calling American TLDs like com/net/org imperialism is too extreme. The internet was developed by Americans for American use and other countries were added to the domain system later. When com/net/org because available, the entire system was completely centralised on the USA so it's not that strange that other countries had to make do with just a national TLD. That's not some kind of attempt of the USA to enforce their rules on the world, that's just historical development. These days, all kinds of TLDs are available, including ones in non-Latin scripts. I don't ever see any .भारत domains on the web (and I wouldn't even know how to type it if I saw it) but it's an available TLD for anyone to use. There are even geographical TLDs like .frl (Fryslân) and .wien (Vienna) that go way overboard with the TLD concept imo.