Rear brake wear. Should I raise a stink?

The scene: 22k miles on a 2010 Prius II. Loud rear brake squeal, worse when backing, but still present going forward at low speeds. Does not seem to go away even when warmed up. I put it off having it checked at first because a little rust on the pads is very common in my climate.

At the dealer, the tech tells me I've worn completely through the pads and scored the rotor! At 22k! On a Prius, for crying out loud! It's not my driving, either, because that's the first I've ever had anything done to rear brakes on any car before 75k. The front brakes needed no attention at all. I challenged the service tech with these points; he even took me back into the shop to show me. The caliper pistons were moving freely, for what it's worth. Ultimately he just shrugged me off. I had the work done and paid the bill.

Subsequent research tells me there's a service bulletin on rear brake hardware for cars in my VIN range, per this post: New TSB for rear brake noise | PriusChat

Seems to me that this is a factory defect, per the service bulletin, and should have been covered under warranty. Furthermore, I'm concerned that if the tech did not follow the bulletin, he may not have made the repairs necessary to prevent recurrence. Would you agree I have a case? I'm terrible at face-to-face confrontation, but I can compose a very effective "sternly worded letter." Who should I aim for first?