No, they shouldn't. You bought a PS3 game and you are entitled to the game on the PS3.
That's what you paid for.
You are not entitled to a newer version of a game any more than you are entitled to the bluray version of a film you bought on dvd.
Funny story, VUDU's disc-to-digital program lets you get an HD digital copy of a DVD you own for a small upgrade fee ($5 to go from DVD to HD, or $3 for a digital copy at the same resolution).
It is actually different with games, as there's generally a lot more work involved in making the ports from system to system. And yet there's several examples of Sony giving us PS4 ports of games for free if we bought them digitally on PS3, even for games many years old. Sure, it's unreasonable to demand every game to be upgraded for free, but that's not even what's being suggested. Things could be better than they are.