The short version is that I never buy insurance, which is where most car rental companies make their money on for American tourists. Everyone else buys insurance. We don't (we book with a credit card that gives us insurance, simple as that).
Every single time we have to go back and forth with the guy at the counter for 10 minutes, before he realizes that we won't buy the insurance, that our insurance is valid, and that we're going to get off with a super-cheap car rental on that day. Shurgs, gives us the keys, next customer por favor.