How is that hijacking? It doesn't stop you talking about everything you want to talk about while allowing others to discuss something quite different if they so desire. If enough people care about what both people are talking about, two or more conversations can happily co-exist within one thread. You seem to see such tangents as being somehow intrusive, while your apparent desire to maintain one-track conversations comes across as oddly exclusive.
Not at all. There are times when you want to shoot the breeze and there are times when you want to discuss something concrete. There is nothing to stop you ignoring a post which doesn't interest you and then start a new thread to discuss what you actually want to talk about.
When you say 'you', you mean 'I', don't you? Why are you trying to impose your rigid conversational rules on the rest of us? When you're in a conversation with several people, do you get cross and shout at them if they start to go off on a tangent? Or do you accept that that just happens to be how normal human discourse works?
Sometimes when I say "you" I mean "I" and sometimes a cigar is just a pipe. If you go to buy a train ticket would you welcome the guy behind the counter giving you his views on the Schleswig-Holstein question instead of a single to Sidcup?