Countries are free to ally with whatever country they want pending any previous agreements
Free to do so, for sure, I'm not claiming illegality, I'm claiming it's wrong. It leads historically to escalation, not to mitigation of tensions. Remember the missile crisis
Eastern European countries made zero promises and had zero obligation to not join NATO
Again, you're not understanding me for some reason. I'm not putting the blame on those countries, I'm putting the blame on NATO itself. It's not that these countries shouldn't want to join a pre-existing military alliance, it's that a supposedly defensive military alliance shouldn't incorporate member countries ever closer to the declared enemy of the US of A.
I wouldn't have a problem with Mexico or Canada willfully joining an alliance with China
I would have immense problems with China fostering military relations with the neighbouring countries of their geopolitical adversary, and if you don't, I think you should rethink that.
Maybe if Russia wasn't such a shitty, untrustworthy neighbor, more countries would be willing to ally with them
I don't want any countries to ally militarily with Russia. I fully understand that Russia has a fascist aggressive government and I'm glad I don't currently live next to it as a Spanish citizen. My whole point is that NATO isn't a "purely defensive military alliance of independent countries", it's an organization subservient to the interests of the USA which has shown no remorse to act on foreign countries which didn't threat military action against member states of NATO, as was the case in Libya and Yugoslavia, and unofficially in Iraq.