I'm going to start off by saying that I used to work at PetSmart.
I'm also going to say that the small animal manager at my store didn't know squat about guinea pigs. (i.e. He told me that my male couldn't get my female pregnant until he was two years old.)
But in the defense of pet stores...
If it weren't for PetSmart and PetCo, there wouldn't be any well-known places to get guinea pigs in my area. (I live in Tulsa, OK and I haven't been able to find any rescues that are dedicated soley to guinea pigs. If you go to the rescues that deal with all sorts of animals, it may take months to find a single guinea pig for adoption.)
Also, if you live in an underpopulated area (such as the small town where I used to reside in Pennsylvania) it's nearly impossible to find a guinea pig rescue. I'd never even heard of them until now. Even the humane society in my small town only dealt with cats and dogs to the best of my knowledge.
And if it weren't for the time I spent at PetSmart, I would have absolutely no interest in guinea pigs. The whole reason I got mine was because I saw one honey-colored cutie in the pig habitat and I started going in to visit him on my breaks. Eventually I bought "Gus" and then a week or so later, I bought his mate, "Maggie" from another PetSmart location.
Also, I've noticed that PetCo does adoptions rather then buying and selling animals from distributers. So even though it's a far cry from a rescue, they're doing their part in helping to find homes for unwanted guinea pigs.
I've noticed that pretty much everyone here seems to be anti-pet store, but you have to realize that just because there are rescues and organizations to help adopt out guinea pigs near you, doesn't mean they're all over. If you're all so interested in rescues, consider starting more of them instead of ragging on the pet stores who sell animals in areas where rescues are scarce or unavailable.