Read the answers given to you. There are different regs in Europe and the US for instance and in the latter it is allowed. So, in Europe you teach your students NOT to do this as it is against rules. Then let the pilots operating under regs allowing this to do their thing.
I spend most of my 6-700 hours per year in the h/v curve in doing precision longline work. During my flightraining I was taught to stay out of the h/v curve. Is my line of work a bad practice then?