Larger flies like houseflies are not only a nuisance, but vectors of disease.
Yes. No one want them indoors.
They fly around and bother humans and animals alike. They bite and spook horses. They leave maggots behind. They are pests because they are pesty.
They can be a nuisance pest, but are generally not included in a service unless you pay extra.
They can find the source but it's usually a joint effort to eliminate them since sanitation and or repairs are involved, something the tech has no control over. Also, the tech has to be very good or...