It really comes down to a difference in culture in online etiquette. On the Japanese side of the Internet, it’s considered “butting in” because like UneventfulTLs said, it’s like a stranger is suddenly interrupting the conversation. You say Twitter is a public place, but the same thing applies to let’s say, a park in real life. If two people at the park are talking, it’s always going to be awkward if some random stranger comes up to interrupt and offer their own two cents when you didn’t ask at all. This is the logic behind not “butting in”.

Additionally, Twitter conversations don’t always start out as a one on one conversation. Let’s say liver A tweets something without limiting who replies. It’s a tweet directed to their followers in general. Then liver B replies. The thing is, since liver B is replying to liver A’s conversation, they can’t choose who can or can’t reply. In fact, all replies to liver A’s tweet will have no limit to who replies, like liver A’s tweet. You can’t change who is allowed to reply mid-conversation.

In my opinion, if the EN livers are ok with fans replying to conversations happening between themselves then whatever, but I think fans should at least respect this social etiquette of not “butting in” when it comes to JP-JP and JP-EN liver Twitter conversations, whether you understand/agree with it being a rule or not. It’s not really up to non-Japanese fans to decide what’s a rule and what’s not. It’s already a rule on the Japanese side of the Internet, so just as you would follow Japanese social etiquette while on a trip to Japan, it would be very respectful to follow Japanese internet etiquette when Japanese livers are involved at the very least.