Mods are the #1 reason some people prefer to buy PC versions of games. To wait 2 years for a PC version and not end up getting mod support, it's not unreasonable to be a bit upset.
Obviously Rockstar never promised mod support though, so I wouldn't be upset at them specifically. Hell, as someone who has bought games entirely because of mods before, I'm not upset to begin with, as it's only a matter of time before the game is cracked open. It will happen eventually.
Rockstar just did what any game developer has a complete right to do, locking their game down, can't really blame them for that. Doesn't mean I don't think it's a bad business decision though, Mods extend the life of your game. It's basically perpetually free fanmade DLC that encourages people to buy your game without you having to raise a finger to make any of it.