from wikipedia.org/wiki/Deep_linking

The technology behind the World Wide Web, the Hypertext Transfer Protocol (HTTP), does not actually make any distinction between "deep" links and any other links all links are functionally equal. This is intentional; one of the designed purposes of the Web is to allow authors to link to any published document on another site. The possibility of so-called "deep" linking is therefore built into the Web technology of HTTP and URLs by default while a site can attempt to restrict deep links, to do so requires extra effort. According to the World Wide Web Consortium Technical Architecture Group, "any attempt to forbid the practice of deep linking is based on a misunderstanding of the technology, and threatens to undermine the functioning of the Web as a whole".