In a nutshell: Sony set the standard for not rebuying things you already own with cross-buy years ago, meanwhile everyone slammed Nintendo because they charged for certain 3DS VC games that you likely already owned on Wii.
The main argument is that if someone owns a PS2 or PS1 classic on PS3, they shouldn't be charged again. Why not implement a system like that? If you have a PS2/PS1 classic in your purchase history then you can download the original game on your PS4, or get an enhanced version at a discount. $15 is a bit pricy for a game that released over a decade ago.
I could understand charging a premium for a full fledged remaster, as everyone's been doing, but these don't appear to be true remasters; only ROMs running at 1080p with trophies.
