To meet Health and Safety requirements, an installer should go part way up the wall and drill it to fit an eye bolt. Then he should extend the ladder and strap it to the eye bolt, followed by donning his hard-hat and safety harness and clipping it to the rope on his ladder. When he's finished he should remove the eye bolt and plug the hole with suitable filler. All of this takes time, which costs money.
Some installers are willing to risk their lives and may charge less because they can do it more quickly without using the correct procedure. My own window cleaner did this once too often and spent a week in hospital, following an operation on his foot, then 6 months in plaster.