There is in every AA hub, but I don't think that is the point of the people mentioning it. Instead of your luggage being through checked, every time you "transit" in the US, you

1) have to go through immigration - something, which, as we all know, can be super quick or a very slow nightmare
2) wait for your luggage to arrive (if "quick" was the answer to 1) )
3) go through customs (not a huge deal, but it still means you need to comply with US customs requirements as well as those of your destination country, and the chance of your luggage being inspected is arguably a lot higher in the US than in the UK
4) recheck your bag after customs - again, that might be quick, or it might just be a far longer queue that you had at check in in the first place, and it typically does not include any special lane for business or first
5) reclear security

This is a far, far, bigger hassle and can represent a far, far longer process than - say, transiting in Singapore where you basically just walk to your new gate where security reclearing will occur. If your flight to the US is delayed and you are unlucky with part of the process, missing your international connection is certainly not impossible, and of course, another difference between most US major hubs and, say, SIN is that the number of options to Australia if you miss your connection is typically a lot more limited, even from LAX which has by far the most in North America.

And if the said delay is due, for instance to a bad weather problem, when flying from Australia (when you are not protected by EC261 or UK261), AA would not typically pay for your accommodation whilst you wait for your rebooked flight (admittedly, the same might be true of a number of non-European airlines if flying back via an Asian route).