I believe some of them do insist on it
The problem is that they assume - unless proved otherwise - that the compromise is on the customers computer, not their system. And given that, they take the stance that its not their problem
Having the stuff installed simply makes it harder for them to wriggle out by using that line of argument. Essentially the argument is, the softwares not on your machine, therefore your machine is at risk, therefore as you can't prove that the compromise wasn't on your machine, then tough