This isn't a cheat or an exploit, it's built into the game & it's always been there. If you share content with another player of any kind (do the same mission, initiate or receive a player bounty, do a showdown, etc.) then both/all players will be placed on each other's 'Recent Player' list and you can join on them in whatever lobby they are in. It works both ways: you can join on them in their lobby as well. The "fix" to this is to shut down the app completely, it will clear your recent player list and take you off of theirs. Some say turning Social Matching off combats this but I don't think it does, there have certainly been times I was followed to another lobby when I had matching off.
Killing someone in their camp isn't a cheat either, it's just something that can be done in certain situations because the game is still broken. I guess you can call it an exploit but unfortunately this is one of many annoyances that Rockstar just won't fix.