Even though I know OP lives in US: in Europe you get written down the notice period on your contract. If you decide to quit or if your company decides to fire you, both parties have to honor the notice period (usually between 1-3 months).
When I want to quit immediately, I don't want to stay miserable in the same job for 1-3 months. 3 months would be insane for me. As long as it works both ways (I can quit whenever, and you can fire me whenever) - it's fine.
You don't reciprocate (as long as you're okay with burning some bridges - which should be fine with above criteria).