Um actually, in the US you can get fired for THC in your system for recreational use. Some states, even if it’s medical.
Yes it’s bullshit, but that’s the problem with it not being federally legal and companies in multiple states. You will probably lose if you fight it, use of marijuana is not protected and your employer is protected from you arguing it is because firing you for this still falls under “at-will” termination.
Only mentioning this to dispel the idea other folks will get away with this. You just got lucky it seems.