Normal working practice doesn’t mean it's a safe working practice that's for sure.
At least it's a learning curve for some not to fall foul of working height regs etc as if anyone had fallen off that roof or through one of those skylights I dread to think what would have happed. Some firms have been reported to HSE by concerned passers by too and been taken to court and fined heavily for their unsafe working practises.
I know it's tempting to cut corners to remain compeditive on price and some think H&S is bullpoope but that is how bad things can happen and sadly do every year.