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.
In the US you can get fired for just about anything in many states, including mine. There are only a few protections in place for things like race, gender etc.
ACKTUALLY, whether or not you can get fired is a state issue, and some states such as California provide statutory protection for employees who use cannabis. It's true that there are some states where you can get in trouble, but the federal laws are not being enforced by the Biden administration, who has given the green light on cannabis consumption and rescheduling, so there's really no single answer that applies to the whole United States, and your mileage may vary.
Exceptions generally are for businesses that receive federal funding, CDL drivers in accordance with 49 CFR Part 382, and non-employees (contractors, students, volunteers).
Still illegal in many places in the US. I just happen to live in a state that is legal.
Since I'm close to a state border, I only need to drive about 20 minutes to be somewhere it's illegal again.
And if you live in an "at will" state, they can just fire you even if it's legal.
Sidenote: "at will" employment is the biggest sack of bullshit that employers convinced people is totally normal.
Im hopeful that the US, EU, and UK legalize it within the next few years. I'm realistic about the chances, I know how the older generations of politicians are, but I still cross my fingers.