Compare joining Gransnet with moving to a new house in another area. If you have just moved in, and walk along your street, you are likely to meet a couple of neighbours discussing something. You say hello as you pass, and they say hello back. If what they are discussing is of no interest to you, you go on your way. If you want to know them better and be included, you introduce yourself. Then you say, "As I passed I heard you talking about ( whatever. ) My opinion on that is ( whatever. ) If your first words to them as a stranger had been on the lines of "You are talking a load of bo****ks", you would not be surprised if they were a bit sharp in their reaction to you.
There are some new joiners (a minority) who do just that - They have perhaps been lurking and not posting, then disagree so violently with what has been said that they must reply, but they forget that their post comes out of the blue, and is exactly how a troublemaker posts messages on random forums to get posters at odds with each other.
There have in the past been a few of those troublemakers. The thread would divide between those who answered the objection with equally strong words, and those who thought that a newcomer was being victimised.