I think you may be misunderstanding something. Active armies were (at least usually, although in the Principate this was not really true) not allowed to cross the pomerium. The moment a serving soldier crossed the pomerium he ceased to be a soldier, and became a private citizen again, and promagistrates ceded their imperium upon entering the pomerium--likewise, even the comitia centuriata could not convene within the city, due to its status as the urban, unarmed counterpart of the full citizen levy. None of this of course prevented a soldier to enter the city outside of the pomerium, and there was no prohibition against former soldiers entering the pomerium after having returned to the status of being private citizens (i.e. after having been disbanded).