There are other industrys which use ladders which alot more dangerous than window cleaning, ok a fall from height is bad, no matter what you are doing. For example i used to do cavity wall insulation, in this you have to carry a large drill that uses a 26mm drill bit, this obviously trailing a lead, and drill upto 100 holes in the house, and as high as 3 bricks under the apex. Yet in the regs they say you cant use a power tool above head height unless on scaffold, but the way they get round it is by saying the cost to scaffold a house is more than the cost of the insulation and that the insulation only takes a short time.
On average 2 and half hours, No houses i clean take that long. Also i think they would insist on a rojak or equivelent on every ladder move!