I'm fairly certain that many people here are misinterpreting existing anti-discrimination laws.
Notably, the Civil Rights Act of 1964, Title II, only prohibits racial, religious, etc discrimination
against customers by "public accommodations" which include hotels, restaurants, etc, but does not appear to cover firearms trainers. If there is caselaw to show that it does, I would be most interested. Title VII prohibits
employment discrimination, but that's an entirely different issue. State law may of course prohibit such conduct; notably California has stricter anti-discrimination laws. However, discriminating against customers doesn't appear to to be generally illegal in Texas.
However, he may well be violating requirements that the state imposes certified instructors, and so may lose that certification. Are there any Texas instructors here who can comment on this?
If I wanted to rent a house but you showed up in a Sarah Palin T-Shirt I couldn't refuse to rent to you based on political beliefs...even though I would want too.
I believe you can. "Political Ideology" is not a protected class according the the fair housing act (1968). Things you cannot discriminate on the basis of are "race, color, religion, sex and national origin..., disability and familial status."
http://en.wikipedia.org/wiki/Fair_housing
All that said, Mr. Keller is still despicable.