itunes extras only with purchase. Why?

[Lynos]

I was very excited to hear back in the day that itunes was adding extras to their movie library... until I found out those extras are only available upon purchase.

I am not interested in buying digital movies, and I don't want to buy the movie just for the commentary track, especially when they cost almost as much as the physical disc sometimes.
Why isn't itunes offering this for rentals as well? I thought the point of purchasing was that you get to have the movie forever and not for 30 days. The point wasn't to get extras. If I'm equating this to a brick-and-mortar video store, then the video store would not force you to buy the disc just to watch the extras. Rental or purchase, you would get the same content.

I would gladly pay a dollar or two more for a rental with extras if that was the case. But 10$ more?

It's not like itunes is producing these extras. They get them from studios. It's usually the same extras that appear on the disc (I know, there are some exclusives, but it's the exception to the rule, is it not?)

What are your thoughts in this matter?