I think they're just taught that all box dyes are crap and parrot it. They genuinely believe it because in the West that's how the industry works.
In my home country (Eastern Europe) you go to a hairdresser for a haircut, highlights etc that you can't do by yourself or they come to your house if they're a friend. However, where I'm from you can find professional dyes in local drugstores and dye the hair yourself if you know how, hairdressers here use a mix of box dyes and pro dyes themselves and will even recommend good ones for your hair based on their experience. Basically they get business anyway because many people don't want to dye their own hair for fear of making it look patchy, they don't give a shit what dye you'll use since they know you'll bring it to them anyway and here anyone can buy "salon brands" so they don't bother shilling anything.
I've never heard any of them say box dyes are crap, just that certain ones are too harsh and poor quality.