You are presuming that they accept all US residents. Nothing could be further from the truth. As a Nevada resident, I found out a year ago that they don't cater to my ilk. Likewise for Massachusetts and some other states. The reason I was told was that it was not cost-beneficial. But according to their website, they allow traders "full access from virtually anywhere in the world", so you should be right.
How does that work though? If they pay 0.001/share and they charge you 0.003/share and you trade, say, 2million shares a month, wouldn't they make $4000/month from you in commissions regardless of where you trade geographically?
It has nothing to do with individual trading volume, but rather the combination of low demand and high state government regulatory fees. Supposedly.