Playing Devil's Advocate.

I agree that it's a ridiculous policy.
BUT...
What is a check if not credit? Isn't it a promise to pay at a later date (when the check is presented at your bank) in lieu of payment (in cash) today?
The FTC has actually given their opinion on this, and they seem to believe there is PP.
From the Coffey letter:
"The dealer may also obtain a report to check a consumer's creditworthiness when the consumer presents a personal check to pay for the vehicle. By contrast, a permissible purpose would not arise if a consumer intends to pay by cash."
[That opinion is in regard to a payment by check for an automobile purchase, but wouldn't the same logic apply to a purchase of anything else by check?]
Here's a footnote from the Coffey letter:
"
1. The dealer's 'permissible purpose' here is provided by Section 604(a)(3)(A), which permits the use of consumer reports in connection with a credit transaction involving the consumer."
Again, I agree it's a ridiculous policy. But contrary to FCRA? The FTC doesn't seem to think so.