I can't remember what the official laws of Asgarnia are, however, I believe Zamorakianism in Misthalin isn't technically illegal, just very, very frowned upon, furthermore, for the Zamorakians to even successfully establish a proper church within varrock itself, must indicate that there's a decent amount of Zamorakians, within Misthalin's populace, that they can't just say no, else risk a state of civil unrest, or at least a few protests.
This may explain why the Zamorakian Herbalist is allowed in Lumbridge. The Guards ultimately cannot actually legally do anything either, since worship of Zamorak is not actually banned.
Zamorakians are hardly even the only other religion in the area as well, Zarosianism, of all religions, is back on the rise in Varrock thanks to Soran, Armadyl recently got a large movement of followers after his little scuffle with Bandos, Guthixians are an outright majority in Taverley (and also in Dwarven settlements) and even a small Bandosian following up in Goblin village has existed un-impeded by the White Knights. Let's not forget the Godless too.
Saradominism is certainly the majority religion, but there's absolutely enough followers of other factions that outright denying them access would potentially cause significant conflict.
It's probably just easier for them to allow it, to an extent at least, as doing otherwise risks seriously harming the economy, potential military entanglements and, worst case scenario, an outright God-war, of which Roald seems particularly scared of provoking.
Also don't forget that Misthalin is concerned that civil unrest could open them up to a Vampyre attack, which would spell bad news for Asgarnia too. Whilst this could fuel flames against Zamorakians, most Zamorakians on the west side of the salve don't seem to care about vampyres at all, best not to give them reason to.
16-Oct-2020 21:46:37