[akhmatova]

Are you required to tell your employer if you're moonlighting or doing other paid work? I'm not.

Actually it's quite common for employment (and even consulting) agreements to have language specifying that you do exactly that -- "devote your full working hours and attention" to their duties for the Company. As well as clauses specifically prohibiting moonlighting (or at least requiring you to declare any such outside relations in writing),

That is: to specifically prevent people from doing what you're doing.

[winterplace]

Then do they fire people who only have 1 job but are coasting?

[akhmatova]

That's a different topic, of course.