If this was so, there would be no 'free kittens' signs in newsagents or on the roadside. Numerous kittens are produced by strays and ferals so moggy kittens are highly unlikely to be in such short supply that they will become profitable. The mindset of moggy breeders is one which does not tolerate neutering or which seeks to exploit an imagined kitten shortage. One oft-used argument of such people is that the public at large won't ago to animal shelters, particularly if they don't live near one. Controlled natural breeding does not, therefore, add to the stray population or deprive strays of homes.
Some people do prefer to go to a pet shop for their pet.
Most pet shops don't interview prospective owners or make them sign neutering agreements. The market to which moggy breeders cater may be one of people who would not withstand a cat shelter's scrutiny. Unless, of course, they intend to supply cats. Researchers usually wants animals with certain uniform characteristics. This has mostly been answered already.