I don't think its common knowledge to be honest, but i dont explain it and i never have since i used it 8 years back i just quote the price to clean if they say yes i arrange to get them cleaned when ever and just do it ... if there in i just say at the end its pure water it dries clear. If there not in then they don't no.
No one has ever questioned it to be honest think there not really bothered as long as the windows are clean, obviously you get the odd couple choosing you because they don't want you walking on there roof tiles or they have hard to reach windows etc.