I agree with your point about a random auto mechanic not allowing you to use his or his employers equipment. I work on cars and computers and other things regularly and if some random person asked to use my equipment to test their stuff, I would feel they were wasting my time, but I am trying to sell a service and I make no money on replacement parts. But, most auto parts stores (even ones with service sections) will test batteries, alternators and starters for you for free if you remove the item from your vehicle and bring it in to the store. I have never brought a starter motor into a parts store/service station and had them tell me to put it back into my car and to bring the car in for their mechanics to test it.
Circuit City/Best Buy have more in common with auto parts stores with service sections than with a repair only auto shop.
Personally, I think it is foolish for a business that sells component/replacement/upgrade parts, which only pros, semi-pros and hobbyists are going to purchase, to not attempt to ingratiate themselves with these persons. In all likely-hood, if a person brings in a component part and it tests bad, that person will purchase a replacement from that same location as long as it is available, besides, these are the same people that are going to make more purchases of repair parts than any other demographic. If they are not interested in this demographic, then they should not waste their floor space with repair parts at all.