That is actually more complicated--
For one, Brits and Aussies grow up surrounded with American media and so they have developed a general ear for the accent to begin with. Hearing it often enough can help with impersonating it.
And, more importantly, with an American accent you really just have to do a general accent and it will be accepted. The US is big and has tons of different regional accents that will often blend together. So US people, in general, will accept something that is just a generic US accent and there's a history for that in US film (look up transatlantic accent).
So an actor can do an accent that isn't perfect and it'll get by just fine.
Many (or even most) US people can do a generic British accent but Brits have a pretty solid ear for regional differences--the accents there tend to be more strictly defined (although I've heard that's changing and there's more and more blending going on). So if you mix in elements on different regions--they'll notice and call it out (I'd also argue that, in general, brits are way harder on this than people in the US are... possibly due to US people just being used to it).
So, basically, in the US people will accept more of a blend and not notice it as much.
Now, if you want to see if someone is actually good at doing a US accent--watch them in a film where they need to have a regional accent and not just a generic US one. I'm from Texas and my dad has a solid twang and he can spot someone who is faking it in an instant in movies.
Regional accents have more firm rules and so inconsistencies will stand out more--actors of all varieties will have way more trouble getting those right.
Hope that made sense--I'm at work and rushed through this to get back to it.