Assuming this is a sincere question:

Geopolitical claims gain/lose legitimacy if they're not refuted. In this case, China's crossing of the Median Line (established in 1955) is to challenge its legitimacy and assert the strait is their waters. If no one challenges that by sailing through the international waters between China & Taiwan then, in the future, they can assert "We did X back in Y and nobody complained then".

America responds to China's patrols with their own patrols on the Taiwan side of the line so such a claim can't be made without refutation.