But what stops them from a: switching HQ, or b: starting in another country? It's not like the US has serious currency export restrictions, right? And little stops a company overseas to do business in the US, right?
I think the answer to both of your questions is, technically nothing. Once you have built infrastructure and a base however those kinds of moves are very costly in time, effort and money.
And little stops a company overseas to do business in the US, right?
I think legally, no, but for better or worse, there are a ton of tools that are U.S. only - an example from today: Microsoft Hololens developer edition is available only to the US and Canada.
At least in the tech sector...because the U.S. currently has, by far, the most venture money [1] and the largest addressable market[2].
[1]http://pitchbook.com/pr_20150109_1.html [2]http://techcrunch.com/2011/12/23/flurry-largest-addressable-... (slightly old)