The argument is that these rules that you describe that any country can join any mutual defence pact without any repercussions is just plain wrong, mainly because the US would be immediately working against that even with military interventions. Its the same thing with how the US's stance for foreign policy is to push democracy where it suits them if they have big influence with one of the parties, and to push favourable dictatorships if not. There's double standards and twofacedness by the US foreign policy which really everyone else sees besides US citizens themselves, mostly because the average american barely even knows anything about domestic politics let alone foreign ones (except the few propaganda topics we get from the three letter tv channels).

Just answer this question, would the US object to, possibly with military intervention, if Mexico or Canada would join a military defence pact with China or Russia, or India, or say really any other country besides the US, even Brazil. We both know the answer to this.

Now lets do even easier. Would the US object to any South American countries joining a mutual defence pact with Russia / China? We already have the answer to this.