Russia is powerless to stop this, since any attack on a NATO country would likely trigger a counterattack.
Hence, instead, Russia must prevent these countries joining if they want to ensure no nukes on their border.
There is nothing threatening about a country joining a defense alliance.
Or you can just beat them up any time they look at other options.
This is a game of “who blinks first”. I doubt normally US would use nukes to defend a tiny unimportant country like Latvia or Slovenia, if under attack. Russia knows this too.
Someone crazy like Trump just might, though.
And they aren't happy about it. At the time, they weren't in a position to do much about it though.
For that to happen they would have to first turn into a tyranical shithole like Russia where an impotent old man can do as he pleases.
They don't start with nukes. They first murder journalists, bomb their own appartments, poison opposition leaders.
They only country in the world which is remotely close to doing a first strike is Russia, and perhaps North Korea.