Because due to a black hat trick called cloaking [1], Google penalizes pages that show different content to users and crawlers. So web sites serve a free version of their content to Google.
Because google will ban the page if the crawled content is not the same as the displayed content when you click their link.
Google discourages websites from appearing in search but requiring registration to view. If the website detects that the user came from a Google search, they don't throw up the paywall.