Why did the loyalist believe that they had to follow the kings laws and rules?

Well because many saw him as a man sent by god to rule, that's why they let the royal family lead for the longest time. They thought the dynasty was sent by god himself, and they were given his wisdom. Or something like that, I got this info out of "Five Equations that Changed the World" which has a bit about Newton's life and what he thought of the matter.