Why doesn't Sony have a movie rental service on the PSN?

[Snah]

I don't understand why Sony doesn't have this yet. They have Blu-Ray, they have individual rentals, and they have downloadable purchases on the PSN...

But they do not have a rental service...you know, pay $15 / month or whatever and rent whatever you wish.

It seems like Sony, being a studio and having longstanding business in this arena, would be one of the first ones to offer such a service on their Playstation 3's.

So, the only question is, why? They could seriously compete with NetFlix in this area given the rising userbase of the PS3. They could even offer the service on their Blu-Ray players. They could even make it better than NetFlix by offering a better library of movies. So why don't they?

Subscription based models are lucrative, and I don't see it necessarily competing directly with Blu-Ray. For instance, I'd like to pay for such a service to casually watch movies that I don't necessarily have any intention on buying, while simultaneously purchasing movies on Blu-Ray if I loved them in the theatre and want them in the best possible quality.

Perhaps licensing is an issue, but wouldn't there be a way to give studios a cut of each month based upon individual user usage data?