I know of a handful of walk ups that don't do foreigners and there might be some I don't know of...
For the ones I know, its mainly because they've had bad experiences - rough treatment / being forced into
doing something they don't want to do / heavy pounding by large pricks that puts them out of business
for a few days ...
Case in point ... recently, there's a bro here who wants to put his prick up the girl's nose
and ejaculate with the intention of watching her go through discomfort motions that this brings
if he persists in doing this, word's going to get around ...
But I think the majority of walk ups don't mind business from most nationalities.