My petco has never given me 'advice' on keeping betta fish.
"Bettas are fine in a gallon of water-they live in puddles so they don't really swim anyway. They just sit there."
My logic? So if I lived in a 9'x9' room my whole life, I wouldn't ACTIVELY ENJOY or DESERVE a 1200 sqft house?
The fish in your little cups are DYING. All their energy is going towards survival, NOT SWIMMING. They swim and they damned well enjoy it, if you take GOOD CARE of them.
When I'm looking to buy a betta from any of the Petcos up here, I put them next to one another to see them flare. Why? Because I want to see their tails. Why? Because every single male I've gotten from Petco has had some sort of fin rot, fungal infection, parasite, tail torn to shreds, etc.
I had one Petco representative actually lecture me that "Flaring is bad for bettas. It stresses them out to see another betta." I said, "I'm checking for signs of illness and need the fish to flare for me." His reply was, "Well, you're going to cause the betta to get ill doing that. If it gets stressed, it'll DIE." I had the cups next to one another only maybe 20 seconds by that point (he was a hoverer).
On another note, the Petsmart up here has an aquatics expert that ROCKS. I was looking at a bowl for a temporary quarantine tank, and he was really worried (he had seen me lookin' at the bettas before and we talk while I buy my plants, etc). He stopped me and gave me a lecture that even one male fish should have at least five gallons of water, and he'd be MORE than happy to walk me through getting the set up and working, and that I should NOT expect to get any fish for about a month and a half going on two, etc. He was really, really concerned. It was sweet. My response? "Darlin', I have a single male betta in a 10 gallon cycled, planted tank. You need NOT worry that I'll put a fish in this teeny bowl for anything other than salt baths."
His look of relief was priceless.