I think they are trying to cover their butts. They don't dare admit the dangers for fear of lawsuits.
When I was looking for a new dentist I did ask about amalgams. Whether they offered it or not. Most of the dentists I asked, don't even offer it anymore. I asked because at the time I was concerned that they were just trying to make money off people by replacing all their amalgams with the white filling.
We seem to have a good dentist now, but we went through several 'bad' ones to get there.
The ADA has viciously attacked dentists who advocate removing amalgam as quacks and exploiters. Maybe some of them are. However, research does show that mercury levels in patients drop off after amalgam is removed, even if there is a temporary spike caused by drilling it out. Anyway, the dental lobby never seems to mention that fillings must be drilled out anyway whenever they deteriorate or need enlargement!