Here's the counter-argument; Knox was voluntarily in the station when Sollecito was being questioned. They hadn't risen to the official level of 'suspect' yet. They started questioning Knox when Sollecito said Knox wasn't as his place for some of the evening. She started providing a statement against the bar owner; they still had her as a 'witness' instead of a 'suspect.' Even the written statement that was inadmissible against Knox was still initially a statement against the bar owner.

All very rational. :)