it is not norwegian, as we would write STOPP in norwegian. In that sense, it's annoying, it's a word you understand, but the sign is also "misspelled".
Good. The purpose of a dictionary is to record how language is used, not to gatekeep or social engineer what is "proper" language and what isn't.
Linguistic prescriptivism is the purview of bigots and school teachers.
It comes in a few flavours. Sometimes an assumption that linguists as people are never prescriptive. Sometimes as an argument for the respectability of established dialect (not to say this conclusion is wrong, only the argument.)
There is plenty of (mostly non-professional IME) "normative descriptivism" floating around.