I wouldn't be surprised if Rockstar is seeding the malware so people don't trust the inevitable cracked game...
The leakers don't just have footage, they have a playable build. One that appears mostly finished and in a relatively playable state. If it was before security stuff was added in, there may be no way to stop people from using it in single player, or even possibly using future versions of this build to cheat.
I didn't realize how big of a deal this might be till this morning, I thought they just had footage that was about to be released, but this could end up costing rockstar billions in dollars.