What if, for some reason, [[ADSL]] would be changed to redirect to another page, or become a disambiguation page rather than a redirect? All those links to [[ADSL]] would then link to the wrong page.
Presumably it would be done when the page is parsed just before being viewed, rather than when it's saved. I guess it would be done at the same time as the existence of the article is checked in order to make the link blue or red.