As someone who used to work for Market Force (and left with no grudges), let me tell you a little more. I have also done some shops for them (after I left) as well as with other MSCs.
First, for those of you that feel like they just "get rid" of shoppers for no reason, that could not be farther from the truth. MF has strict agreements with its customers (like their fast-food client) that not only require precision and accuracy but also a lot of mystery shoppers on the bench. Quality is essential for the customers, but quantity is crucial for MF. There is no reason why MF would purposefully get rid of good mystery shoppers.
Second, there are analytics done behind the scene of results, the sole purpose of which is to not only spot mistakes but biases that could jeopardize quality results. For example, if a person did 30 shops and generated high scores on all of them, this would trigger a flag if the remaining population generated high scores on only 70% of them. Again, quality is of crucial significance. There are other analytical metrics as well that can identify perhaps unknown hidden biases.
Mystery shopping is a competitive business, and the number of competitors is huge. It is a difficult business to run, as you have to deal with everything like unreliable shoppers, very demanding customers, low profit margins, etc. The strength of Market Force over its competitors is its shear size, but that can also be a double-edged sword. As many have experienced in this forum topic, one of the disadvantages of a low margin business is the ability to have well trained shopper support that can efficiently and effectively answer questions for shoppers, including why some get disqualified from further shops.
Finally, remember that mystery shopping is fairly low paying and low skilled work. With this comes mystery shoppers that do not provide the same quality and concern of others, and upwards of 80% of them will complete a shop and never do another one again. It is the 5% that consistently perform quality work and shop after shop that are the most valued by these types of companies.