Every country has their own interests - they can join whatever they want. But keep in mind that every action has a reaction. If you share a border with Russia and you join a Military Alliance that is hostile towards Russia, well get ready to get a reaction to that. I don't know if you remember the Cuba crisis? Well the USA didn't like that, but why? Everybody can do what they want, right? And btw that was a reaction to the USA putting Nukes in Turkey (i believe) back then, which could reach Russia. What do you expect would the USA do if Canada and Mexico would join a Russian Military Alliance that is hostile towards the USA? You think the US would just sit and be like ''Ahhh just let them be'' - that's delusional.
Russia and USA are not friends and putting weapons on each others borders is not a safe thing to do. Simple as that. I would also not like Russia putting weapons on USA's borders, because that just crosses the line, which the US is doing right now. You have to keep a healthy balance - which right now is not the case, which is the reason Russia and China want to create a Multipolar world.
The world is not a Utopian place, you have Big World Players - USA, Russia and China - who pretty much decide what's going on and the smaller players either join one side or the other. That's how the world works. You can like it or not, but that's the simple reality.