Ok, I'll try and be more precise here in what I meant with the above:
As you can see they did fine him for the violation of the rules, so an explanation like "Pls excuse me I just forgot it in my room as I'm not used to wearing one in my home country where I've been practicing before I came here." or something similar won't justify you violating these new rules, which I think is a perfectly right approach by USTA.
So I was not trying to say they didn't do the right thing in fining him. They simply have to be strict in this respect in order to have everyone following the rules, no matter what reason there may be behind the violation.
I was only trying to explain that while I consider myself a bit too pedantic and diligent in all matters related to avoiding getting Covid-19 these days (I was already infected once early last spring when no-one over here knew to be that careful yet, and even that wasn't directly due to anything I did myself, I just had a friend who was spreading the virus around without knowing it himself - and even though still pretty young and perfectly healthy, a former athlete who still exercises a lot, I got a pretty nasty version of the disease -, I'm taking all precautions I can now that we're all much more well aware of things as I don't want to go through the disease again, let alone spread it around to others even though I'm no longer infected but naturally not tested for it all the time), I still made that sort of a mistake myself as well.
So, what I was trying to say is that the pro tennis players, just like the rest of us, are only humans and they make mistakes just like the rest of us do, even if they try their best to avoid making them. You simply can forget things like this without being careless at all.
I remember Andy also telling in one interview that he at least once left the court without his mask on, having just forgotten about it altogether, but luckily someone noticed it right away and just told him to put the mask on. He didn't know how long it would have taken for him to notice the lack of it himself.
And I'm 100% sure that these three cases that we are aware of so far have not been the only ones. As long as humans are involved they do make mistakes, even if they'd prefer no to (and no, machines, computers etc. are not error-free either). Just a fact I've learned in life.