I am certain that mystery shopping companies and clients are fully aware that there is the potential for cheating and that cheating occurs.
The onus is on the mystery shopping companies to make sure that they are providing the clients with honest, reliable data. When they fail to do so, the egg is on their faces. At the same time they are not paid enough to do thorough background checks on every shopper who registers and competent shoppers are not paid enough to make mystery shopping a valued career choice. My best year mystery shopping I earned before expenses roughly 25% of my pre-retirement salary. "You get what you pay for."
Most mystery shopping companies address their honesty issues with a variety of fail safes from trick questions to extensive proofs to similarity between shoppers of shopper findings to try to weed out those who are dishonest. I found several years after the fact that I was not seeing shops from a particular company because they had decided based on my description of the restroom that I had not visited it and had falsified the report. They didn't ask me, they paid me for the shop and they just didn't let me see further shops. It hurt and was annoying because I thought I had a good working relationship with them. I certainly had no memory of the restroom in question, though I know I would have reported what I saw. Reviewing my records, I had only two shops that day, both had restroom visits, so the only thing I can figure is that perhaps I switched the restroom observations between the two reports. I know I did not set out to falsify anything.