Is it an AC vs. UA thing, or a US carrier vs. non-US carrier thing?

I've found US carriers even encouraging of taking the earlier flight, or of taking a more direct routing (as long as you don't want it, it's free -- as soon as you want it there is a change fee ).

My few attempts at trying to fly earlier on non-US carriers (both in US and in Europe) would have required a change fee at the minimum, and an entirely new ticket at the worst.