ALL OF MY NUMBERS ARE MADE UP AND JUST FOR EXAMPLE!!!!
sony takes in money for the subs...
they turn around and offer some of that money to third parties to put their games on plus for free. (either in a lump sum before putting it up or based on how many people download it... I haven't a clue which way they do it)
if there are 2 million ps+ users (I haven't a clue how many there are) subbed for a year at $50 (the normal sub price... the actual average would be lower) that is $100 million for sony to use to get games on PS+ (as well as fuel discounts).
if you were a third party and in the last 3 months your game had sold 1k copies at $40 each (fo 40,000) and was slowly trickling down to nothing, and sony came up to you and offered you 500,000-1,000,000 to put your game up for free on plus... would that not be a decent thing to jump on?
sony gets around 4-5 games a month for plus... taking 5 games a month over 12 months nets you 60 games. Even if sony offered 1 million for each game in this case that is only 60 mil of the 100 mil. The end result in this case would be a service that makes the subbers happy (in general), the third parties getting a payout for games that likely aren't selling well anymore, and sony being able to pocket 40 mil extra dollars