I mean, I think it would be fairly easy to narratively justify PvP in the current situation.
Both factions are still tired from the War, and have an official armistice. This is agreed upon between the two factions, and provisions of that include not sending armies into rivaling territory, not breaking out in open conflict, etc.
However, there can be a lot of ‘grey’ areas in Azeroth where neither faction has a solid claim (or both factions have a claim that was not resolved in the Armistice as achieving overall success was deemed more important than every ‘minor’ territorial dispute). Old tensions are still high, and there are a lot of wealth, influential, and hawkish enough people on both sides to still be able to fund small skirmish efforts.
To some extent, both factions are aware of these bad actors moving around, but both factions, in the interest of diplomacy have decided to silently close the matter. Refusing to send official supplies, soldiers, or material to any of those conflicts. Neither faction is officially participating, however small groups of motivated irregulars (i.e Adventurers) continue to help supply material and personnel to these conflicts, though in small enough numbers to keep the involved conflicts to little more than skirmishes. Things that can be safely ignored by the larger calculus of superpowers such as the Horde and Alliance.
The conflicts are happening far enough away from any ‘critical territory’ that no faction wants to risk the entire Armistice to argue them, and the back and forth sway of them keep them all locked in some form of stalemate. Skilled healing magic, and the Irregular’s own extreme resilience keep actual casualty counts extremely low, and both factions (quietly) have decided that the cost of raising an international issue, one that could end out in another World War, is too great compared to the insignificant impact of the skirmishes.
Still for many of the actors in such conflicts, the ability to make their enemies bleed, if only a tiny bit, is well worth the effort. Not to mention that engaging in such skirmishes keeps the Horde and Alliance’s elite honed to the deadly edge that makes the factions so fearsome, not to mention the Honor and Glory that can be won engaging in such affairs.