It is unethical NOT to A/B test, because you may be doing the worst thing for all patients. You're right that it should be generally known this is done.

I certainly agree for trials for treatments for specific ailments. But insurers, hospital administrators, and others have always been making decisions that balance clinical outcomes and cost of care without patients knowing such a decision has been made. The experiments I refer to are similar to those choices and typically the funding will go testing choices that haven't been made because they are more costly in the short run.