Book stores do not violate copyright law when you read a book on their premises without paying for it. Copyright law specifically concerns who is allowed to make copies. Yes, TPB can serve the same purpose as a physical library, but the law doesn't care about the market role of the infringers (mostly). It cares about whether or not a copy was made. That's why it's legal when libraries and bookstores let you read something you didn't pay for, but not when websites do the same thing. Accessing a library does not create a copy, but accessing a website does.