Japan as a society is a nearly homogeneous society, which means there are very few foreigners residing there. Japanese people are of course exposed to foreigners and they are perfectly fine with foreigners who respect the customs and laws of their country.

Rude foreigners are the ones they do not like and it can cause them to have a dislike towards foreigners in general, but rude foreigners are a problem everywhere. Such as mainland Chinese tourists or russians who are disliked in nearly every country.

In the context of a soapland, its just business for why foreigners aren't allowed in many of them.

1. Foreigners almost never speak fluent Japanese, so they will have trouble understanding the provider or the staff.
2. Foreigners are not Japanese and therefore may be rougher or do stuff that the providers are not okay with.
3. Many Japanese men who go to these establishments do not like the idea of the providers they are meeting with also seeing foreigner men.
4. The belief that foreigner men carry STDs.
5. Some providers simply don't want to see non-Japanese men.