If you are a Cop then you put up a business in your town which gives you bigger amount of income compare to your salary, then I think it is still consider as a side job. For me, the main reason for having a main job is its security. You are secured to have an income even if your side job fails to do that.
That is right, the main job is just more of a security, every month you’re sure of earning a salary from it. The side job thing is basically just a way to more of assist yourself and make extra income to further assist yourself. Just like you have said, someone who is a cop is definitely going to be working more as a cop and committing more of themselves to being a cop than they would to their side job, because side job would be something that they are doing when they have a free time.
But, people who are lucky are able to get a side job that would be making them more money than the salary that they are getting from their main job. It’s not bad to maintain different sources of income, regardless of which one pays the most.