Did they even ask you if you are a US citizen/resident or you got a benefit of the doubt?
Do you need to pay them with a US debit/credit card?
Are you sure it will work outside the US (in terms of getting SMS)?
As far as I remember, I paid with EU debit card, but cash payment is also possible. They did not ask ID or anything like that. They definitely sell to anybody.
Online top-up (recharge) also works with EU cards.
I use the sim all the time abroad, works fine in few EU countries where I have tried at least.
Interesting detail was that the sim card itself didn't come with number, but when activating the card (online) they asked your zip code and number was then assigned accordingly. I bought the number from NY, but gave NJ zip, got 201 number (NJ).