What you're saying is that your kid asking to pet somebody else's dog is merely a pro forma social nicety. That your child is entitled to get all grabby and handsy with someone else's property. INSANE. There are a million reasons why someone might answer no when a stranger asks to grope their dog, many of them totally valid. If your kid couldn't handle the disappointment of being told he has to keep his grubby little hands off of someone else's property, then you HAD ZERO BUSINESS TAKING THAT KID TO ANY STORE. 

You don't know that the dog doesn't play well, you don't know why the owner took it there but didn't want it petted, but home depot says they can. If your kid can't keep its hands to itself, you should NOT BRING THEM IN.