The court ruled that at the time, when the State Police opened the file, they had no reason to believe that a warrant was required. While the search was later ruled unconstitutional, no court had ruled it was unconstitutional *at the time of the search*. One of the cornerstones of American jurisprudence is that you cannot go back in time and overrule decisions based on contemporary jurisprudence.
From the opinion: 'the exception can also apply where officers “committed a constitutional violation” by acting without a warrant under circumstances that “they did not reasonably know, at the time, [were] unconstitutional.”'
If you're interested, the discussion of a good faith exemption (and why fruit of the poison tree doesn't apply here) begins at page 40 of the doc.