The main point of a grace window is to give ICE agents a choice where quitting is the best option, as opposed to incentivizing doubling down.
If it works immediately, they'll just go "welp, nothing I can do about it now, might as well stay with ICE and keep collecting a paycheck since even if I quit it won't matter!" vs. "damn, maybe I shouldn't take the risk and I should get a different job now instead before that 30 day window closes in on me"
Personally, I'd like to see a mix to have some immediate punishment and some optional (if they quit) punishment too. (e.g. an extra tax on all your income for the next x number of years if you were an ICE agent at any time during the stated period, whether you quit later or not, plus being banned only if you continue your employment there)