Not sure of the reason, but I believe a US resident can simply open an account with the euro-division if s/he wants to use the other platform (there used to be a commission difference, but now they're both $30 per mil USD).Quote from snackly:
We're sure of that? That really sucks if so. What would the reason be for that?
They should just offer both platforms to clients of both divisions (as it's the same underlying ECN), but they don't.