For the parts of interest here (machined motocross components), if they carry the claim of "Made in the USA", then they must be made from US made material (scroll down to "Are raw materials included in the evaluation of..."):
Complying with the Made in USA Standard
Regarding the origin of the 6061 material (and associated temper), it really shouldn't matter where the material comes from so long as it is actually tested to the appropriate standard and passes. So if you're getting material from a reputable supplier, then it's largely irrelevant. The question really is how reputable is your supplier? Most major US suppliers are reputable. Beyond that, though, how reputable is the manufacturer? Are they really making the components from what they say they are made from?
And regarding the differences between 6061-T6/6511 (the most popular aluminum for machined components) and 7075, there are many things to consider. First, most people don't realize that 7075-T6 has issues with stress corrosion cracking, which hardly makes it ideal for something under constant stress like a hub. The over-aged T73 condition doesn't have stress corrosion cracking issues, but isn't as strong as the T6 condition. However, it is stronger than 6061-T6 by about 37% and harder by about 30%, though hardness isn't really relevant here.
So, relevant to hub design, 7075-T73 is the best choice from a material standpoint alone. But you have to think of material cost and material availability. These two factors alone usually make 6061-T6 the right choice, but beyond that, if the part is designed with 6061 in mind, then 6061 will work just fine at a lower price point and high material availability, all with minimal difference in weight for the same strength as 7075.