It's possible that they already have enough overseas workers and want to focus on winnowing out the poor quality workers and outright scammers before they allow more in. If they have half a million people on tap already but only 150K - 360K HITs total, that means that if everybody did just 1 HIT there'd still be a third of those 500K stuck without a seat when the band stopped playing. I suspect their active Turkforce (new word!) is both much smaller and biased much more heavily towards overseas workers than their official numbers let on. I read somewhere that they have 46% US and 36% Indian Turkers (which makes sense, since those are the only two countries where you can actually be paid in cash), but I'll bet the number of active Turkers is probably a fraction of that number and contains a much higher percentage of overseas workers, given the types of work that are actually offered on MTurk. I'm thinking probably 10% -15% of registered Turkers are actually active, which gives them a total workforce of 50-75K, and I think that probably the split is more like 70%-30% to 80%-20% in favor of overseas workers. Using these estimates, (which I freely admit that I pulled from my posterior) that means you have 35K to 60K Indian Turkers and 10K to 15K US Turkers.