I've rented from Thrifty SLC a few times. I've never had an issue with Trip Saver being added to my bill but I have had a few other disagreements with how they run their operation. On my last two visits the folks at the counter asked lots of questions.
It started with insurance. The desk agent wanted the name of my insurance company and my policy number. I understand why a company that rents cars would be interested in this info, but this Thrifty office is the only place I remember being asked for it.
Next, they wanted to know where I'd be going. As in, names, addresses, and contact phone numbers of the places I'd be going. At that point I objected, stating it was personal information they had no rights to.
"Management requires us to ask," the person at the counter explained.
"Will Management refuse to let me sign this contract if I don't answer?" I asked.
"No."
"Then please let your management know that I, as a customer, object to being asked these prying questions."