"Thread hijacking" is considered SPAM?

"For example, User A posts a help thread asking about how the shelter works, when User B posts 'This is happening to me too!' and User C asks about how to evolve Jirachi. Both B and C are going to be looking at mute pages once a moderator finds this or has it reported to them." Why are the messages sent by User B and C in the example thread considered spam and calls for an instant mute? Why don't people like these kind of messages?

A moderator should really answer this for an official stance on it but for why generally people don't like it would be that user B didn't answer the question and user C was off topic. I'd be annoyed seeing 2 responses to my question, when neither are useful. It's easy for user C to start their own thread for their question regarding Jirachi. User B was writing something pointless and could have just kept quiet about having the same question. They could have upvoted user A's post if they felt like showing some kind of solidarity with it. I often upvote good questions/answers to things, as a way of showing I agree with what's said or that they've posted something useful/good/informative.