Joining an email discussion list isn’t like joining a forum. All of the old messages don’t automatically appear in your inbox. And I don’t tend to keep all old emails I get. Most are irrelevant. The vast majority, I’ll never look at again.
There is one email discussion group I participate in using a gmail account. I’m not sure why, but conversations are frequently broken into multiple threads by gmail, and they’re very difficult to follow. Messages from one thread are quoted in another, and chronological order is difficult to maintain. I’ll take a message board any day.
Just as often, it means, “I agree with most of your post, but I take issue with this part.”
There’s a question you might ask yourself, when you’re feeling picked on. “What else can this mean?” If you’ve “always taken” things as slings and arrows, you may often be mistaken.
I think you’re taking it the wrong way, and chances are 99.9% of people who do this do not mean to imply this in any way. If you are addressing a specific point it does not make sense to have the entire post repeated over and over in the thread. It is already there, right in the same thread, and chances are it was already read. The smaller quote serves as a pointer into the original post to indicate what particular point is being addressed.