Aussie Bob, you do not shut off fixed wing engines using magnetos
Err, Gipsy Major with no mixture, Rotax, Jabiru? Err, why not?
I will cease the method on real aircraft engines and take yr rights opinion as correct. By the way No Hoper, this is a regular practice with some seaplane operators just as yr right said. Personally I never did care for it that much, but if you are restarting instantly and you use the very low idle set on most seaplanes first, perhaps it is not as bad as you think.