They make the assumption that someone will disrupt it or spoof it. Same for Galileo and GPS. Might be them, might be someone else, but they're so easy and effective to spoof/disrupt that it'd be stupid to assume they could rely on any such system.
They also suspect that other people might make use of it, and that denying its use to those others would be a good idea.
This is legit also because it’s already happening in the sea/boats, but it’s the same for every other position system, maybe they put less trust in Glonass? In this case every states/military should use a hidden/unknown position system because the “official ones” could be hacked.
I don't think that GLONASS, at least the military-grade stuff, can be hacked or spoofed, but since radio frequencies are well-known any adversary can just pump enough noise to make the system unusable. This vulnerability is not limited to GLONASS - any satellite positioning system is vulnerable to "pump too much noise and it's unreadable without space-grade stationary equipment", which is already demonstrated by Iran versus satellite television and radio.
They also suspect that other people might make use of it, and that denying its use to those others would be a good idea.