When an employee signs their employment contract this will include a detail of their remit, (job they are required to do). I doubt any of Iceland's employees would have cleaning the windows written into the remit. However, if they are not part of any union then they would have to fight this themselves. Most would just give in and go out and clean the windows to keep the peace and their job.
It's tough on commercial windies but high street retail shops are really struggling to make a decent profit
in this day and age. Supermarkets are slowly choking them all to death and we just have to look elsewhere to find work. This is just the way it is today and will only get worse and worse. So just go find some good rich residential if you are wise.