The situation is rather obvious. A reasonably large bunch of people who are legally unable to represent themselves as a group can have the rules applied as the taxman sees fit so they cannot enjoy the normal tax benefits of being overseas for sustained periods. In Germany, for example, I believe the NAAFI staff do not pay UK income tax.
On the other hand rules that they might benefit from, such as television licence requirements for buildings, can be worked the other way so they do not benefit. HM prisoners don't need individual tv licences because they do not have occupant-controlled locks on doors. I'm sure a decent lawyer could argue uninvited military room inspections makes individual's rooms public property?
The bottom line is if you're a government needing to raise taxes to pay MP's mortages, why not sting the forces with every tax available no matter where they are - what are they going to do, complain?