I'm no lawyer, but I'm thinking it works along those lines.
I invite you into my house, let you use my bathroom, while in my bathroom you take the piece of art sitting on the wall, the prosecuter would likely accuse you of being a thief.
Hank didn't have permssion to search, nor sieze Walt's private property. Nothing about the book directly implements Walt in a crime, so he couldn't be arrested on the spot, the book would not be permissable in court.
It does have me wondering, breaking it up like they have, if small details like that are going to be left out to bring the show to an end, that appears to have more conflict than just Walt via Hank. I kind of wish they would have spent more time on hank combing thru the details, getting fustrated by finding nothing of value that links Hank to Gayle other than the hand writing. It didn't have to be long, just one of those time lapse sequences that they did when they were cooking meth several times, kind of highlight his fustration of thinking he found a huge break in the case, but failing to find any additional connection back to Walt.