Because you can't get a warrant without probable cause. That's why it's illegal for the police to just arbitrarily decide to get a warrant and search your house

Because you can't get a warrant without probable cause. That's why it's illegal for the police to just arbitrarily decide to get a warrant and search your house for drugs because you look like the kind of guy who might have drugs. And the police can't say "Well, we didn't have a warrant, but we were right so who cares" either. If they never had probable cause to get images of this server, what gave them the right to a warrant for it? More importantly, the world needs to know how the heck they did know, so that we all know what the government is capable of. Ulbricht is still an idiot, and it's not surprise he got caught eventually- but we can't let the police bullshit their way into convicting anyone, even idiots.