no that's a new one to me, and don't think its a legal issue even if you were, might even be considered a rights violation for asking or not selling if you did say your not a US citizen
ammo can legally be sold(were and what type) is legal to all over 18 or 21 in the USA as I know it to be
some city's have special reg's and towns, but over all ammo isn't on the ban list anymore(Black talons and such compants just stopped making them LOl)
doesn't bother me if they ask either
hey no big deal if you ask me, but ??