I think the argument is that the information found on the server shows that Ulbrecht was running this. It'd be like if I was using eBay to sell drugs. I don't own eBay, so I couldn't claim my 4th ammendment rights were violated by a search on eBay.

I don't understand how this isn't an illegal search irregardless though (maybe the server hosts gave the FBI permissin? I'm not aware of the specifics)