At my store we have a restroom that we allow the public to use, but I know of many stores that don't. There are times I have seriously wondered why I do. We get people that stop there just to use the restroom and leave. That wouldn't bother me so much, but there have been several time that an excessive amounting of cleanup is required after some of these visits...
And this is the reason for the issue we are discussing. I have seen this trend come along over the years and also understand the reasons for it.
But if an obvios customer asks to use the facility the business should comply. A homeless freak not so much.