According to the law, they are to be paid the prevailing wage. That is a slippery definition, because I think it means the median wage, but in some situations it means the highest wage.

Either way, the point is not to depress local wages.

The fact is, H1B is being abused by body shops that are paying lower wages. That must be stopped, not only because it depresses US wages, but also because it's illegal.

US wages for software engineers, even sub-standard ones (wages, not engineers!), are still more, and in many cases much more, than most countries in the world.

I was making more as an intern in the US, that now as an experienced engineeer in southern Europe, and that not considering income taxes (much lower in the US).

But doesn't everything cost more here too, which offsets it?

Is income the primary/only motivation?

No, it does not cost more. Things you use every day like cars, fuel, clothing, etc usually cost considerably less in the US than in other countries.

And you might be surprised, but in the US "foreigners" are usually treated much better compared to other countries where immigration is not as common, like Germany or UK.