UPDATE 11/8/15 5.32pm: Rockstar has offered the following explanation as to why it's banned the folks behind the FiveM mod (via Kotaku):
"The FiveM project is an unauthorised alternate multiplayer service that contains code designed to facilitate piracy. Our policy on such violations of our terms of service are clear, and the individuals involved in its creation have had their Social Club accounts suspended."
UPDATE 11/8/15 8.45am: FiveM's use of the Rockstar Social Club login does not guarantee ownership of the game, Eurogamer has been told by a user familiar with the mod.
Instead, the hookup to Rockstar's service was included as part of the mod's identification system, used to determine unique players on FiveM's dedicated servers.
But even without a user check, why did Rockstar ban those behind it? The developer has yet to comment.
ORIGINAL STORY 11/8/15 8.45am: A group of Grand Theft Auto 5 modders have been banned from the game after creating their own multiplayer service.
The account suspensions come despite the group's attempt to comply with Rockstar's rules, which state modding of the developer's official GTA Online mode will face the banhammer.
So modder NTAuthority made his own multiplayer service away from GTA Online, in order to tinker around safely. Alas, Rockstar's banhammer has struck down him and other users anyway.
The mod project, named FiveM, has been active now for several months. It allows users to play GTA5 with custom multiplayer modes and separate servers from Rockstar's own. It also allows users to produce map mods.
FiveM has been used to bring basic versions of the maps from Grand Theft Auto: Vice City and GTA: London into the GTA5 engine, for example.
Its users insist it does not mess with the official GTA Online at all, and so it should be exempt from any action by Rockstar itself.
The GTA developer has previously set down its own rules for PC mods. Simply put, if you are only altering the game's single-player mode then you are fine. Mess with GTA Online at all and you risk your account being suspended.
"We have always appreciated the creative efforts of the PC modding community," Rockstar explained earlier this year. "To be clear, the modding policy in our license has not changed and is the same as for GTA4.
"You should not worry about being banned or being relegated to the cheater pool just for using single player PC mods. Our primary focus is on protecting GTA Online against modifications that could give players an unfair advantage, disrupt gameplay, or cause griefing."
FiveM's users say that the mod falls under the scope of a single-player project as it only uses the game's single player code - albeit with their own online add-ons.
However, part of FiveM does entangle itself with it Rockstar's servers, even if it does not technically touch GTA Online.
"Here's the ironic part," FiveM user Ash Rogers told Eurogamer. "FiveM uses a Social Club hook in, it did this to make sure PEOPLE OWNED THE GAME, as a piracy protection method so people couldn't just crack the game and still get to play MP.
"FiveM is the only thing out there on the GTA modding scene to do so. Because of this piracy protection check, Rockstar have got angry and started to ban people. YES, people who CONFIRMED THEY PURCHASED THE GAME!"
Rockstar accounts are permanently linked to Steam accounts, so any affected user becomes banned from all Rockstar games which require a Social Club log-in - including GTA5's single-player mode.
It may be that Rockstar's anti-piracy tech is simply flagging up a mod that interacts with Social Club as an attempt to modify the game's online portion.
But, for some, it is enough to suggest Rockstar is targeting the mod and its creators specifically.
"Rockstar complained about piracy in the past," Rogers concluded, "so when modders make sure people actually own their games, they get angry. Rockstar complained about wanting to keep modders away from GTA Online, so when modders make their own third party client, they get angry and ban them.
"Even when modders comply with what Rockstar wants, they still get kicked in the teeth for it for having the gall to attempt to expand the game."
Rockstar has yet to comment on the case of FiveM.