> Russia does not get to tell other countries they aren’t permitted to ally with each other.

Logically, no - they don’t. In the real world, it does not work that way. Hence, the US has the Monroe Doctrine. It’s also the reason why US has invaded Latin American countries and many other ones. “Might is right” is an unfair unfortunate realty in international military situations.

Americans have superpower bias. They think that every country can ally and antagonize any other country they want. When you're a buffer state, you don't have that privilege. If Mexico started talking about a military alliance with Russia, they would have some American freedom missiles delivered very shortly. Ukraine is Russia's Mexico.