As a project maintainer, you shouldn't make rules unenforceable rules that you and everyone else know people will flout. Doing so comes makes you seem impotent and diminishes the respect people have for rules in general.
You might argue that by making rules, even futile ones, you at least establish expectations and take a moral stance. Well, you can make a statement without dressing it up as a rule. But you don't get to be sanctimonious that way I guess.
Except you can enforce this rule some of the time. People discover that AI was used or suspect it all the time, and people admit to it after some pressure all the time.
Not every time, but sometimes. The threat of being caught isn't meaningless. You can decide not to play in someone else's walled garden if you want but the least you can do is respect their rules, bare minimum of human decency.
Everything has a trade-off.
On the other hand, it also prevents companies from dumping artificially cheap and crappy goods (TEMU) on US markets and making it nearly impossible for others to compete.
Unsuspecting consumers buy a super cheap (subsidized) crap product on Amazon or Temu or Shien or wherever - probably a knock-off of an American product, have it shipped to the US, then it disintegrates after a couple of uses or stops working, and we wind up with pollution, additional landfill, and relentless consumerism that's harmful to the country all so we can help a certain country whose name starts with a C keep the lights on and keep factories running so that they don't see unemployment numbers tick up.
Legitimate businesses selling higher quality products where they exist will be able to figure it out. Or not. It's not a big deal if your sunscreen is slightly worse than the Korean version (which I use). Maybe it just hasn't been approved because they haven't done the work to apply because they can get around working with our government and making sure their product meets our safety standards because of the de minimus loophole?
There's also safety concerns, which I think the CBP did a good job of overviewing here: https://www.cbp.gov/frontline/buyer-beware-bad-actors-exploi... . Send drugs or guns or illegal animal products to the US, get caught, who cares you live in (not the US) so you can just spin up another sham company and do it again.