They include the answer to the question in the page so Google and other pages pick it up. You'll often see part of an answer in the blurb under the link in the search results.

When you follow the link, it hides the answer from you and basically tries to shake you down for money to get access to it.

Individually the practices are ok. When dealt with together, it is a nasty business practice.

Alternate explanation: Google "experts exchange sucks"

Alternate explanation 2: If you first found the site while looking for an Expert Sex Change, you might still be bitter.