Question: Why would "not quitting" be a headache for employers? I thought in the US you can just fire people you don't want anymore, instead of waiting for them to quit. Makes no sense to me.
Only in "at will" states, and usually then, only when your company is small enough (like <50 employees). And even then, you still have to be careful to not give a reason. If you give any hint of a reason, the employee can sue for it.
And the reason can be as simple as “no business need to continue employment”. And you can sue for anyone for anything at anytime, but it does not mean you will get anywhere, except losing money on court fees or lawyers.
The only direct monetary cost to employers is increased unemployment benefit insurance premiums due to having more previous employees collecting unemployment benefits.