MITM attacks are only possible if you have a privileged location in the network: "the middle". Further, it is easily possible that neither the people operating Site A nor the people operating Site B have access to the SSL certificate for this domain name, as they might be behind a proxy or load balancer operated by the people holding the domain's SSL certificate. You can't just randomly MITM someone else's traffic: you need all the pieces together.

Your assumptions about what is possible in certain hypothetical situations should not cause all security opportunities to be eroded. I mean, seriously: the logical conclusion to your argument is that all non-SSL websites should just share a global pool of insecure cookies, because it isn't like any of it can be considered secure anyway. The people you are protecting yourself from matters: these attacks are different threat models, and are secured separately.