Gotcha. I think a lot of people are not familiar with ATF regs on interchangeable ammo, and certainly not Dick's managers: ATF Online - Firearms - Frequently Asked Questions - Conduct of Business (Licensees)
I was not aware of that exception to the rule. An odd exception if I may say so. I'll bet the Dick's corporate lawyers have said "..no sales to minors...", and that is that as far as the managers are concerned.
Or, it could be someone who is not informed, like myself.
Or, it may just be some dick working at Dick's!
Whatever, reps for educating an old man.