I don't do these shops, but I have to disagree with the OP on several points.
1. A reimbursement absolutely can be considered pay in some instances. I agree in all instances it is not, but in many it is. I do gas station mystery shops that pay $6, reimburse $5 in gas and $1 in an in store purchase. I consider that an $11 payment. Why? Because I have to buy gas. It's a requirement for my vehicle to run. And I only buy top tier gas so I would be buying from a major brand anyway, and they rarely vary more than a few pennies. I do not count the $1 in store reimbursement as pay. Why? Because I wouldn't have purchased that item. It's an expense. In the case you're talking about, it's groceries. A smart shopper would buy items that they were going to buy on their normal weekly shopping trip on this shop. Which means it can be considered pay because you would have purchased the item anyway.
2. A useful reimbursement (gas, groceries, oil change) actually has significantly greater value than it's reimbursement. Take this particular grocery store. For me it's an 8 mile round trip. If I did this shop once a week I would have 416 miles to deduct on my taxes. If I don't do the shop, I have to go there anyway. So, now I have an extra $225 to take off my earnings for the year. That's a $28 savings in self employment tax and anywhere from $22-$90 savings on income tax. Let's count it at the lowest and say I've just saved $50 between the 2. You also have the $9 reimbursement for all of those shops, which is $468. Here's the thing. It's more than that. Going back to that tax rate, let's say you are in the 10% bracket. To make $468 after tax to pay for those groceries you would have to profit $604 on other shops. Because that income is taxable. So now your profit is an extra $134 plus $50 for a total of $184, which is just over $3.50 per shop. Based on that the $14 is actually worth $17.50. Of course it could be more or less, based on how far away your store is.
3. For SOME shoppers this could be even better. Let's say you are retired and drawing social security. You have a part time job making $15,000 a year and also mystery shop part time. At $16,920 your SSI benefits will be reduced. So you need to keep your MS income under $2000. Taking 1 of these shops every week would allow you an extra $600+ in "profit" from MS without lowering your SSI.
4. The "practice" is not insulting. It seems most shoppers that do these shops can complete the report and the extra time in the store in about an hour. $17.50 an hour is a rate that is acceptable to many shoppers, especially when factoring in that some people just seem to like them. I will almost always do shops that I like at a lower rate than ones that I don't like.
Now, having said all of that, I do agree with the OP in that I don't do the shops. For several reasons. Mostly I just don't like doing them. Even if I did like doing them, I don't normally shop when I am home unless it's a shop I particularly enjoy. My home time is downtime for me, not work time. On top of that, my threshold for work is higher than $17.50 an hour.
There are reasons that a body stays in motion
At the moment only demons come to mind