After of its activation almost three years ago, from time to time, I keep hearing on the news about Russia making statements about the NATO missile-defense launch system placed in Deveselu (locally known as "Deveselu shield"), namely that it is a threat to Russia (source and source):
"Both the US and NATO have made it clear the system is not designed for or capable of undermining Russia's strategic deterrence capability," US assistant secretary of state Frank Rose told a news conference in Bucharest. "Russia has repeatedly raised concerns that the U.S. and NATO defense are directed against Russia and represent a threat to its strategic nuclear deterrent. Nothing could be further from the truth," he said.
The United States says it is in "full compliance" with the Intermediate-Range Nuclear Forces (INF) Treaty, the landmark Cold War-era nuclear accord.
The statement from the U.S. mission to NATO on February 8 came a day after Russia demanded the United States destroy a missile-defense launch system deployed in NATO member Romania in order to return to compliance with the INF.
The Romanian Foreign Ministry also had a reaction to the latest Russian statements:
(..) the ballistic missile shield at Deveselu is a strictly defensive military structure, explaining that his remarks come in relation to the Russian Defence Ministry having asked the US to destroy the Mk-41 launching system in the composition of the Deveselu shield, on the grounds that it would violate the Intermediate-Range Nuclear Forces Treaty (INF).
From a layman's perspective it is quite hard to understand why a defense system would be a serious issue for such a large military power.
Question: Why is having a NATO missile-defense launch system such a big deal for Russia?