On one hand, these policies (just like the Chinese-only policies of many flushing houses) are prejudiced and discriminatory.
On the other hand, this isn't selling coffee, it's a service that involves a physical connection and therefore providers have a right to discriminate amongst who they want to see.
Having talked to various MMS over the years, these policies exist because they limit robberies (common at AMPs), attention from LE, and troublesome customers who disturb other guests by arguing over which services are being provided or whether they are being shorted time.
They fall under the class of a typical stereotype - based on anecdotal experience, but statistically, more true than not.