The threat of warranty denial is something that has been used for good and ill for nearly 40 years.
A couple of random, unassociated thoughts:
1. The relatively low cost of carpet in someone's home makes a battle over warranty coverage not worth anyone fighting over. In the commercial field, it is a more significant issue.
2. As a one time carpet inspector, I can tell you that the wording of warranties are such that few claims are settled in the buyer's favor.
3. The opportunity to damage carpet exists with most any method. I've seen more damage caused by improper brush choice and/or lubrication with a
CRB (approved tool) than with an OP/scrubber/Cimex (though those too can cause damage if improperly used.) The burden of proof in a warranty claim isn't just the method used, but also proof that the method caused the damage. For example: If a red stain doesn't come out of nylon, the fact that an OP was used in cleaning isn't really relevant. If tip bloom is the cause of the claim, then PERHAPS that could be involved in the claim.
4. I will contain to rail against the "pH of less than 10 is ok" part of stain resistant carpet warranties. There are a number of "less than 10" products that have alkaline builders and solvents that wipe out the stain resistance anyway. That compromise was made out of practical necessity, not scientific proof.
5. I'd have loved the "18 month cleaning" to have been a strongly enforceable issue, but I never saw that as a sole reason a warranty was denied.
I could go on and on, but if Marty has read this far already, he's getting even more annoyed with me than usual.
I'll sum up (sorry Marty, might be long...)
The fear of voiding warranties by improper cleaning caused a spike in training, certification, and the paying of closer attention to the cleaning products cleaners used. The value of the education (it is hoped) exceeded the threat, and a lot of cleaners did a better job as a result of what they learned. I can tell you that a vacuum cleaner on the truck didn't become something that was paid attention to much before that training spike in the late 80s, as an example.
So I guess it is like this:
Cleaning related warranties were like Santa Claus:
Cleaners behaved better out of hope for reward (customers who would come to them to maintain warranties) and fear of loss (terror of replacing carpet wherever they cleaned).
Neither came true, but behavior improved as cleaners waited starry eyed for Santa Claus to deliver them success.