hi, if the bus shelter has adverts on it then these are usually cleaned by the company who own's them, round my way they are adshel, often see the same guy cleaning them, he is employed by adshel, they (as far as I know) don't subcontract as he also puts the new adverts in each week or so, the other shelters without adverts are all owned by the council and are cleaned by a local window cleaning firm using wfp.
Will depend where the bus stops are as to how much cleaning they need. Most near me are never that dirty, but ones in city center's are going to be worse than one's out in the countryside.