Because the Russians claim that those same missile silos can be used to launch offensive missiles at them.
Oh? The Russians full well that these are limited to defensive interceptors, and their location in Poland makes them no threat to most Russian ICBMs which are clustered along the trans-Siberian railroad in eastern Russia.
Poland is along the great circle route from Iran and the Middle East to the USA and Europe. That's why the missiles will be there, not to target Russian missiles whose tracks are thousands of miles to the east, from Siberia over the North Pole to the US.
You have to stop reading the Washington Post and the New York Times.
“Because the Russians claim that those same missile silos can be used to launch offensive missiles at them.”
I know, but that puzzles me that they would even try it considering they are likely to be granted the right to inspect the silos.