What law requires him to be in business is the real question.
Sadly, in the great State of Texas, you have to have a government permission slip in order to be allowed to exercise your 2nd Amendment rights. That's part of the need for firearms instructors, and why many people seek them out.
I'm sure those Muslims who wish to get firearms training will find someone who is willing to do it. I'm not sure I would want to do it, either, honestly. I believe, however, that you can refuse such services to someone who has intent to commit a crime with them. I know one gun shop employee who made a potential customer leave when he explained he was buying the gun to shoot someone (and no, he wasn't joking).
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