It was legal because in the king's eyes, the colonists weren't really citizens, they were just subjects, and therefore subject to search and seizure on a whim.

It was legal because in the king's eyes, the colonists weren't really citizens, they were just subjects, and therefore subject to search and seizure on a whim. That was one of the reasons we had a revolution. There was also a brief period between 1789-1791 where it wasn't explicitly clear that it was illegal, but that was resolved by amending the constitution.