Ughhhh, again with the petty "waaa f*ck rockstar, greedy f*cks!" nonsense. I'll explain to you why Singleplayer DLC is technically not possible with the current DLC system.
Grand Theft Auto V uses a file in the update.rpf named "dlclist.xml". This XML document is used to mount DLC archives. All DLC is mounted using this file. Grand Theft Auto Online enforces file consistency (albeit very poorly, it just checks to see if the RPF is signed, nothing more) to make sure all players have the EXACT same archives. If archives do not match, you can only play with people who have the same exact files as you. This could be easily tested before Rockstar blocked using dinput8 (ASI loader) and OpenIV.asi in GTA Online - you would only be able to play with people who had the exact same mods as you. (However, you can still get online by signing your RPFs, but I suggest you don't do it, modding in GTAO is a no-go and will lead to a ban).
So, if SP DLC happened, players would be segregated by who owns the DLC or who doesn't. Now, instead of cutting up the playerbase, they could make the DLC a forced download and have it be unlocked after you fork up some cash, but nobody would want to download a large DLC package of content they can't access without paying. Another idea is that the DLC could be a free update, but there is no business sense in that. The ONLY WAY to have SP DLC is to chop up the playerbase - but this would kill the MP. DLC packages like map packs do the same thing to other games that have multiplayer, this is exactly what killed Max Payne 3's multiplayer..
So to all of you saying "f*ck rockstar" over something like this, grow up. The only time we should say f*ck rockstar is when they hurt us directly (OpenIV's short-lived shutdown is an example). Not getting some DLC isn't gonna destroy the damn game.