The US is happy to fly an F-22 around willy nilly in air shows and whatever /s

The real answer is that the J-20's RCS is probably similar to the F-22 and they realized that the J-20 is vulnerable to this. This has been a known problem with stealth technology for forever, so it's really more of a deterrence. China really doesn't want a war, which is why their Navy is so heavily oriented towards coastal (defensive) operations rather than blue water (offensive/power projection) operations.