There is a school of thought in the cat grooming industry that says anything is acceptable, as long as it doesn't result in death or injury. I find that way of thinking abhorent as it completely ignores the psyche of the frightened feline.
The cat may have arrived terrified & if so, the owner needs to be counseled on how to work with the cat to bring it in a calm state to the salon. If not possible, a house call groomer should be retained. If the cat becomes agitated & frightened upon arrival, or can't be calmed down once there, the grooming environment needs to be re-evaluated. There are many factors that come into play including, noises, smells, lighting, placement of carrier, location of grooming table, energy & handling methods of groomer & staff, medical issues, & more.
It should be every groomer's first priority to work on building a friendly & trusting bond with the cats (or any animal being groomed). It may take more than one session for some but the result is beneficial to everyone involved. Restraints are counter-productive and delay the building of trust. It takes more work & time to get to this state but slapping restraints on a frightened cat is, IMO, inhumane. Personally, I don't use any form of restraint, including scruffing & muzzles. Respect & affection go a long way with cats.
Groomers would benefit from reading up on the latest thinking in cat psychology & behavior and not rely on outdated thinking & methods, just because they or whoever taught them has been doing it that way for decades. How long a groomer has been grooming has no bearing on whether they're doing it humanely.