Hi - I'm from the UK too, and here, waiters and bar staff are all paid at least the minimum wage, which is a decent amount to live on.
In the USA they're often paid really terrible wages - certainly NOT enough to live on.
I agree that the problem is that the USA doesn't have a good minimum wage level, but until that changes, unfortunately everyone is expected to tip between 15% and 20%. The Americans have all got used to it - and I guess that when we visit the USA we just have to accept that as their tradition and go along with it.
It's like having to pay more at a supermarket than whatever you've bought was price-marked, because of their sales tax being added at the till. I find that really annoying! But it's just something you have to deal with when visiting the USA.