PSN Plus should let you play demos BEFORE they are released on other platforms, and not create an artificial "24 hours exclusive" by just delaying it one day for the people that doesnt pay.
If PSN Plus didn't exist, probably it would have been released for al PSN users the same day as in other platforms, because they wouldnt want to be the last ones releasing it.

So its an "added" content, but they substract it from the non PSN Plus users. And they said before implementing PSN Plus that non paying customers wont have anything substracted from the services they were receiving.