As a developer here are my thoughts...in case anyone cares. 😛
First, they expire after four weeks so they are not "freebies" but more for reviews only. As a developer this is great but I only see it good for reviews...testing would be for before it hits the store and this still has to be AdHoc.
Why 50? Why not, do you really think each version needs 50 people to download and try it(Review it)? I doubt it, I think they are being generous for the high end apps.
Developers can't charge for them...and this is another reason they only offer 50 and they expire. If they didn't then every developer would sell X versions of their website and Apple would loose their cut...not a very good business move. 😉
Why US only? Apple is based out of the US, it's easier to do all the legal stuff here first so if they are testing something or slowly releasing stuff it's easier for them to do it here than anywhere else. Plus the majority of the developers and downloads come from the States so why not start off in the largest market?