A lot of these people are on Google Groups where they see
whole threads.

The rest are just clueless.

There's a page on Google where you can ask them to implement
quoting as default behavior. It's a simple form to fill
out.

Google groups used to auto-quote. That stopped being automatic awhile
back. I would suppose that is accounting for the upswing.

To "quote" from google groups, you need to click on the show options
"option" in the post you want to quote and then click on the blue
"reply" link at the top of the box that opens.....It's explained in the
Help section of google groups....