What's to keep the same low-lifes from replying, "I agree
too, Dude", and re-directing it back to r.r.a.a?

Because they are clueless to how Usenet works. The message they are
replying to is NOT in r.r.a.a, it is now in some other newsgroup, and
there are let's say, 50 of them, so many that it is an annoyance to
even read through them. So when they finally have had enough after
reading 10 of the 50, they decide to reply, their stupid reply will
only go to the newsgroups that are in the message they are replying to
(which does NOT contain r.r.a.a). Ok? Got it?

Rick