China did not just fly planes on its coastline. They crossed the median line, which is an unofficial line that has been dividing the Taiwan Strait for decades. Planes and vessels from China and those from Taiwan would not cross this line to show mutual respect. China is purposely breaking this unwritten convention. See how they usually just barely cross the median line, fly parallel to the line for a bit and head back? Neither are the planes passenger planes, they are fighter jets. This is different from the US sending ships through the Strait. Sending a military ship through the Strait is a provocation to China, but it is much weaker than the direct provocation of the fighter jets crossing the median line.