Most importanly, I think, this now-common Internet convention was adopted in order to circumvent (or simply to mock) Net Nanny-style blocking software, like those set up by "concerned parents", public schools and universities.
When these software packages originally became popular, webmasters feared that any mention of the word "porn" on their pages would get picked up by automated search routines, and some adopted this alternate spelling just to keep their sites off the ban-list.
As it turns out, this is probably a bit of an overreaction, since the algorithms used by these applications are usually more sophisticated than a simple one-word text search. Still, the habit persisted and soon became second-nature.
