You're right about that, but what would have happened if Ulbricht had banned and removed every post, deal or business that seemed to him (whether it was or not) to be illegal? I suppose Silk Road would probably have fallen into oblivion shortly after its birth. But they are still very different cases.
Deleting would not help a lot as agents infiltrated, recorded and monitored everything.
Back then Bitcoin trading was not legal or illegal, it was obscure thing used by small number of people and almost nobody expected it would become much more valuable in future.
Interesting that in whole process some government agents stole a lot of coins and they thought it was all anonymous and private.
Basically everything you write in the website is public, and saved somewhere.
Even this forum is scrapped by PX-Z. Everything you post here, no matter you delete or not, is saved in his server.
Imagine Twitter, bitcointalk, reddit, etc... everything you do is recorded