And the store would be immediately sued for violation of customer's civil rights, regardless of the merits. It was the right move to use BATFE rules as an overt reason to deny the transaction. The seller is not required to hold a trial and to determine facts with absolute certainty. A prima facie suspicion is sufficient. Now the store is on the right side of the law.
I don’t think political affiliation is considered a protected class for commercial non-discrimination.