Presumably because when they see the sticker they'll know that you called them without shopping around, so they'll quote you something ridiculous and bank on you not realizing you could easily get a second opinion.
Ah, the vendor doesn't run a generic repair shop? This makes sense if the only people that will call the vendor for a repair are the people who had the vendor install the thing in the first place.
They don't have to know the sticker is there while on the phone, only by the time they give you the quote. Once they show up at the house to diagnose the problem they'll be able to see the sticker or not.
To be clear, this is just me speculating about how such a scam would run, I've never seen it or heard of it.
