There are any number of reasons why your friend could have told you that. The stock is extremely limited (sometimes boring), the conditions of the fish aren't always up to par (sickly, stunted, or worse), the cost may be too high for certain species compared to a private owner, etc. Heck, some people won't buy fish from any store that keeps betta's in little cups.
A lot of hardcore keepers tend to shun the chain stores simply because they feel they have transcended beyond that stores capabilities once they start acquiring oddballs and monsters. Strange mind-set, but everyone has their issues.
Most of the time people just don't like the uninformed employees of a chain store or the way they go about keeping their stock.
Some of these reasons are good, some are less than good, but it really depends on how you feel about your local Petsmart. There are three Petsmarts near me and only one of them has healthy and high-quality fish worth buying. The species offered simply do not interest me.
If you like what's there and the fish are healthy... keep on buying! Just make sure a local fish store doesn't have better fish for cheaper prices.