Please note that a "link" in this context is a direct text hyperlink to another website, not framing web pages or hotlinking images (which many web sites obviously hate).
A "deep link" is, according to Wikipedia, "consists of a hyperlink that links to a specific, generally searchable or indexed, piece of web content on a website (i.e. http://example.com/path/page), rather than the home page (i.e. http://example.com/)." Surprisingly enough, many websites (mostly large ones) prohibit such linking in their TOS altogether and do not allow others to link to any page besides the home page. The majority of pages are in fact deep links, meaning that an entire web site is off-limits to linkers simply with a single blurb in the TOS.
Additionally, many other websites (again, in their TOS) seem to think of linking as a "privilege" and specifically give permission for users to link to their site, often with restrictions such as "non-commercial use only" or "we reserve the right to ask you to remove such links to our site for any reason".
Is disallowing deep linking really a common practice, and what is the intent behind doing so? From what I know, having others linking to your website is a good SEO tactic and encourages more people to visit your website (and click on your ads, buy your products, etc.). Also, I haven't seen any cases where a site has problems with search engines indexing a web page meant for the public to see (like a product page).
Why do many sites seem to like doing this? What is your take on people linking to your sites? Not having a link does not stop the Internet community from talking about how bad your site is....