I feel like your original phrasing is a little ambiguous. To say they are a scam sounds like they are fraudulent, but they are not (as you clearly understand), they are in all but a handful of cases a necessity.

Of course they are scams:

It is illegal to sell insurance by non-licensed sales people - see Zenefits. It is illegal to claim that something is insurance or describe it as insurance when it is not. If paperwork says it is "waiver" but someone who makes me sign for it says it is "insurance" they are committing a crime and if a company actively encourages such behavior the company is engaging in a criminal activity

All I'm saying is if anyone reads this and is one day standing at a rental station considering the liability waiver and remembers someone referring it to a scam, they might wholesale dismiss it. People do it all the time because there seems to be a widespread belief that credit cards cover everything.

Insurance, waiver, whatever, it will save your ass if you purchase it when you need it and one should seriously understand whether they need it or not. It's that value that makes me question whether one should call it a scam.