Accessing a computer system without authorization is a crime. Facebook is the only entity that can dictate the terms of lawful access to Facebook systems. If Facebook's terms dictate that you cannot access their systems using another person's credentials, then doing so is unauthorized and unlawful. I don't know how well that would hold up to the various appeals processes, IANAL, but there seems to be some case law supporting it for now.
Update: 2 of the comments on the ACLU article say this also. I'm surprised this line of reasoning isn't more prominent on the issue.