What about a subtler case? You begin working as a US based employee, but after enjoying your work and proving your mettle, you take the risky choice to move abroad without informing your company? Or, you have a US based apartment but periodically for a month here and there operate abroad unannounced?
Additional wrinkle: while the expectation is more clear informally, it's not clear to me what US based employment is from a legal standpoint. Is it merely filing your taxes from a US based address?
In the very slight chance you are already doing this (cannot tell), you need to come clean with your boss immediately, preferably face to face.
TLDR yes you can get rid of an employee easily but the costs vary depending on with cause vs without cause.