First of all, ER vets charge many more times the price normal vets charge during regular office hours. The only time going to an ER vet makes sense is when a pet's life is in immediate danger and lifesaving measures to save the pet's life can't wait.
Also, as far as some vets are concerned, the life of an older pet has no value. They are like the physicians who don't care enough to provide quality health care for elderly patients.
However, all this still goes back to one simple fact. If an owner tells the vet don't diagnose, don't treat, because money will not be spent on the pet, that's exactly what the vet will do. The vet can't do anything else because his/her hands are tied.