How come most sites can get away with such crappy treatment of users?

The NYTimes wrote a little while back about how hard it is to delete an account on most social networks and web services. I got to be a voice of dissent (and reason):

Still, not every site takes the “Never Gonna Give You Up” approach. Alexis Ohanian, the co-founder of the social news site Reddit, said that if users wanted to delete an account, “they should be able to do that as easily as they signed up.”

“It puts the onus on us to keep delivering a great product, and not retaining users simply because they can’t find the exit,” he said.

But really, why are sites like reddit the exception, not the rule? How valuable can 'trapped' users be? How much are you slacking on your product and business because you don't have to keep earning that user?