Swartz accessed a system he did not have permission to access. He did so by entering a restricted area he did not have permission to access. While I agree with his general ideal, what he did was an act of civil disobedience.
The single most basic tenet of civil disobedience is that you are willing to go to jail.
Swartz was apparently unable to think beyond the immediate and to the inevitable, obvious, end result of him being charged with a crime that he blatantly and admittedly committed.
Six months house arrest, which given that he had no criminal record, would likely have been halved or reduced even further. This is absolutely in line with the crime that he, again, admitted to committing.
Mitnick's sentence shows exactly what these maximums mean in regards to real-world sentencing.