The forum is private property. They can do whatever they want. All you can do is leave.
According to laws in my country, any webpage or forum accessible through computer network is a public space not a private property. Denying access to forum is like forbiding people to enter shopping mall based on skin colour or religious views.
Deleting Silk Road thread is not from outside pressure (such as a gun pointed to admin's head), it's forum owners own ill advised action. Today you delete Silk Roat thread. Tomorrow you will insert blacklist functionality into Bitcoin client to block known Silk Road seller addresses. All censorship comes in small increments.
Take a example from TPB. Instead of sucking goverment sausages, release Bitcoin - Silk Road edition with link to Silk Road .onion address on the GUI to show your attitude. Just one-time compilation like VLC Player 1337 Edition. Simple as that.
