Getting a float rating should include flying curved approaches and takes off. It has to because as you have noted rivers are rarely straight. But really twisty rivers are really twisty! In comparison the Thames is all but straight.
The simple fact is that official-dom is unable to deal with these sorts of things. The jobsworths are terrified of making a decision and that means it is much easier to just say no.
Compare with Victoria where float planes land in the harbour - alongside the government buildings and directly adjacent to the channel. And why not? Any half competent pilot should be able to land on a runway and that is considerably narrower than any water landing area. Once you are waterborne you are simply another - somewhat vulnerable boat.