Edit: To to clear, DPR could have well assumed it was the same party and figured this was one way to negotiate, instead of saying "ok, how about $200K instead of $500K"?
I guess it depends on whether you're factoring in the "on trial for attempted murder" angle.
After all, if friendlychemist did in fact have information on thousands of drug dealers, he ought to be able to get a pretty good asking price from the government, so why would he even contact Ulbricht in this scenario?
What DPR should have done is had his team of admins hellban the guy everytime he tried to spam the forums with names so he could waste his time however DPR never seemed to give a shit about the forum even though it was his brand being represented there.
This is why Max Vision spied on all his buyers/users because if they ever tried to extort him in the same manner he would promptly reveal that he's been inside their systems for months and knew exactly who they were.
The game theory of scam artists is really simple: your expected probability of scam N+1 working goes way the heck up if you've successfully worked that mark in scam N. That's why you can literally find people selling lists of folks who've fallen for 419 scams.