you're probably right M.Ted that the stuff I've heard may not be note for note identical, just sound close enough that you can't tell at one listening - but I'm not really looking at this from a legal ownership trying to claim copyright point of view.
It just strikes me as rude/cheeky to copy something that closely and not acknowledge it. There's no way, in the examples I was listing, that the performers could have come up with those arrangements independantly and 'just happen' to have them sound so close to a recorded (and often well known) other version.
As an amateur muso myself I tend to listen to things quite analytically and am likely to think 'hey, nice arrangement' if it's something I like. I always feel a bit cheated if it is a copy of something else - and there must be times where it is a copy but I don't realise it cos I don't know the original.

I know that plenty of other people who don't listen analytically like that, which is fine, but the arrangement will still have a big bearing on their appreciation of the music. Especially if the audience are not aware of it, it can lead to a performer being quite highly rated when actually they are ripping off other people's hard work, which sets my teeth on edge. Yes there's more to performance than the arrangement, but it does count a lot.