The same reason the Soviets threatened pre-emptive strike when we built anti-ballistic missile interceptors.

By definition, if you can defend yourself from attack, then it threatens MAD, and therefore weakens the incentive not to launch a pre-emptive strike.

Then again, why would we be the ones doing that when Communists have always advocated genocide and nuclear war as a moral imperative to destroy Capitalism & The Bourgeoise?