To address the OP's points; I think there is some confusion between treating shoppers like humans, and treating shoppers like employees. The complaint appears to be mainly based on the abrupt rejection seeming inhumane and unprofessional.
Humans make mistakes, and employees mistakes are often forgiven, due to the nature of the relationship. It's less work and expense to retrain and forgive employee mistakes than to replace an employee.
I worked at one point as a true employee doing mystery shopping and though I pride myself on being an excellent shopper, I accidentally shopped the wrong location of a store due to a clerical error on my part. I was paid for my time, scolded and asked to shop the correct location as soon as possible. That's when a large component of the employee vs. IC status really became apparent to me.
As someone who has been a contractor almost all of adult life, I was shocked that I was paid for the useless data I submitted to my employer. I was half-expecting to be fired for the mistake when it was pointed out to me. Upon further reflection, I realized that after all of the paid training, drug-testing and other time/expenses the company had put into me, it was certainly worth $50 to them to sweep my mistake under the rug and move on.
However, almost all of us in MSing are working as contractors. As a contractor, the responsibility to perform the assignment correctly lands on you. It is specifically professional and businesslike to decline a shop that was not done to the required standards. This is a business-to-business transaction so asking to be treated with humanity goes both ways.
Yes, the MSC can potentially spare the time and expense to reach out to failed shoppers, explain what went wrong, why the shop could not be accepted and gently inform them they will not be paid, but that same courtesy could also be extended from the shopper business. Where is the expression of shame in not providing the required completion of the assignment to the MSC?...and the understanding that human beings at the MSC now have additional work and expenses to be handle because the assignment was not performed correctly?
I'm just saying that it's a two-way street. If you make a mistake, own it! Apologize to the MSC and promise to do better next time. And if a MSC makes a mistake and declines a shop for invalid reasons, feel free to make stink and demand that they do the same thing. Seems like in this case, the OP is kind of admitting the mistake was on their end, however.