I am closer to siding _with_ Garadget in that as a private business, they don't HAVE to provide a service to someone (and it looks pretty obvious that there was no particular discrimination or the like); in fact, they have the right to refuse service.
That said, the person also DID purchase a physical item (in fact, Garadget probably should have refunded them first, just basing this off my own personal ethical point-of-view).
That said, if a negative Amazon review IS the real reason (it looks like there was a bit more to it), then this COULD also backfire as bad publicity. Do other people want to give their business to someone who in the past has refused service to someone solely on the basis of a bad review?
I've left negative Yelp reviews and tweets before in earnest to those that I think deserve them, and while I genuinely won't be giving anyone repeat business after I've been prompted to review them, I would be even more upset if I _was_ refused service on the basis of my reviews.
As you can see, businesses seem to really try to find out who has personally left negative reviews, so I'm sure they all know who I am. That said, I personally don't swear in any of my negative reviews, I try to keep them constructive.