This case has next to nothing to do with "being anonymous online." This woman was stupid enough to contact a woman on a message board, set up a meeting with her, and then murder her at the meeting. Stinnett was on the phone with her mother when the murderer arrived. The cops could see that the last thing Stinnett did alive was answer the door to the woman from the message board.

In these circumstances, obviously the police will obtain a warrant for your computer. But if she hadn't murdered Stinnett, she'd have been anonymous. The moral is not "you are always traceable online," but as Chaucer would say, "murder will out."