1. Like they said above, the lawsuits happen anyways, there's no bulletproof-enough waiver, and OBGYNs already have the worst malpractice rates out there, or one of the worst.
2. The thing about BTL is that it is very effective but there is potential for it to become less effective over time if the tubes grow back together. This tends to happen years down the line--iirc the dip in the graph happens about ten years in. Now if you are doing a BTL on a 36 year old woman delivering her last desired child, by the time you hit that ten year mark, her fertility is going to be crap anyhow and it's very unlikely that she will have a problem. But if you do it to a 21 year old, she will still be quite fertile when that dip in effectiveness happens. Also younger bodies heal better, so there's a chance it might be more likely to happen in the first place. IIRC younger women also have a higher risk for dangerous ectopic pregnancies after a BTL for the same reason.
3. There are several options for female contraception that are comparably effective, more readily reversible, and safer to implement than a BTL. Namely the hormonal implant and several different types of IUDs. The arm implant is the most effective form of birth control for anyone short of hysterectomy. More effective than vas or BTL. And easily reversed with a 5 minute outpatient procedure, and practically foolproof. Women who have cv risk factors that prohibit taking the combined pill can use it. There are very few people who cannot use it.
Now when there is a safer, cheaper, more reversible, more effective option out there, a patient who comes at you screeching that you'd better give her the worse option is going to come off like an ignorant harpy, right?