Ok, so in my opinion you shouldn't be able to mark inside regions, why you might ask?
Because it gives no benefits to either party, the attacker nor the defender. It might give the attacker's side some fun, but the heavy nuisance the defender's side experiences heavily outweighs the attacker's ~fun-time~.
The attacker's party has already been inside the defender's territory, and has most likely already gotten most of what they can get out of the area. In which case, marking and coming back later will yield close to nothing.
TL;DR
Marking inside towns gives a slim chance to gain something, and is thus mainly used to camp and annoy the fuck out of people for a little ~fun-time~. Marking inside towns is therefore heavily outweighed by it's actual use, making it nothing but a nuisance.