Good enough for Ozzy in Sabbath:
Singers do as they please, especially if they're decent. Guitar players are easily replaceable. Bands are supposed to be fun.Certainly a cringy move, but I feel like it's going a little too hard to instantly write them off as a band you would ever gig with again. They don't know your preferred stage dynamic at all, and I'm sure if you just told the singer politely to refrain from doing this in the future he would be fine with it.