Hi John
Yes I agree. As a singer, on many occasions I have been put off singing by musicians, who at first quietly and then a bit louder, joining in the tune of a supposedly unaccompanied song. I have sometimes felt that I was singing along with the musicians who start controlling the pace and volume of the song. That's obviously not on!
Having said that, not many musicians are that insensitive! But once or twice when it does happen and I decide that I don't like it, I have walked over to them in the middle of a song and wagged my finger at them!! It works! But it is a bit aggressive, I'm afraid.

"Three jolly coachmen sat in a Bristol tavern ...!"
Alan