They got a collective bonus from selling Michelin or Michelin owned tyres a few years ago. They also got bonus from trackings per month and extras sold whilst changing tyres (balancing, valves, high pressure caps). They are not the worst though, and I am particularly critical of Quick Fit's sainted adverts in the last few years when it was they who sold unnecessarily, and plundered the replacement damper market. They made motorists very sceptical to various things even when they are clearly needed.
All these fast fits try to sell what is best for them. Even independants and distributors usually have about 2 brands where they recieve a larger discount. When I was in business, I resisted that type of temptation to recommend what I genuinely believed would best suit the customer's needs. Tyres were not a major part of the business though, and we probably only sold 800 per year.