As said, the real reasons why R*/Take2 wanted Open IV shut down are unclear and we can only speculate.
I'm not the most advanced modder here, nor do I have any inside knowledge of how R* works. However, I do have a reasonably senior position in a multinational business. I get in on occasional meetings with the top brass, and I do think I've got something of an insight into how big companies can end up making bad decisions that aren't good for them or their consumers in the long term.
Whilst I respect a lot of the theories about the reasons, and I still wouldn't completely discount them, having thought about it a lot, I think the decision was in fact driven (however misguidedly) by cheating in GTAO. Here's WHY:
In April as most people here will know things got out of control, with the Force Mod menu allowing the remote manipulation of other players rank and GTA $s. R* attempts to stop it with cheating countermeasures failed within days. From a reputational standpoint, they had to take some sort of action.
It appears today GTA Force Hax has been shut down, with a note on their website stating they have been in discussions with Take2. They have issued an apology and stated they will be donating their proceeds to charity. Regardless of how sincere that all is it was clear these goons had no plans to go anywhere - they've blatantly had the squeeze put on them.
It's just too much of a coincidence that this happens within days of the Open IV team also getting a C&D order.
Of course, along with many here, I've been previously discounting cheating being the reason, because those of us in the know are well aware that Open IV has not been the cause of GTAO's problems. Surely R* couldn't be that ignorant, right?
Well, this is sadly where some corporate experience suggests to me this kind of mistake is entirely possible. Here's HOW:
Issuing a C&D legal order is not a trivial matter. Any business issuing legal action has to consider each case carefully (from a legislative perspective) - if you dish these things out willy-nilly you can end up in a whole heap of trouble. So, someone senior at Take2 has to sign off on each C&D order that is issued before the legal department draft it up. (And by the way, while it'll be someone high up, it's almost certainly not going to be SZ, the Housers or anyone you'd have heard of making the decision).
The person signing off may well not have a development background (at lot of senior people making such decisions come from finance in places where I've worked). So while they will probably have taken advice from people more technically astute as well as legal, they probably have very little idea of what tools like Open IV or the more malicious mod menus do, how they work - certainly much less than everyone posting in this thread.
I can imagine that post April they'll have ordered a review of all major GTA modding tools on the internet to decide which ones were threats. The Force Menu will have been an open and shut decision - it can manipulate GTA $, everyone involved will have agreed that needed go. I suspect there may have been more of a debate regarding Open IV. Just because a company makes a decision, doesn't mean everyone who works there agrees with it. That's not to defend R*, it's simply a fact of the way things work. Some developers may well have spoken out that it's an important community tool that is much loved and used to produce content that benefits the franchise. However, I can also imagine when a non-technical exec is given a description of what Open IV can do, they may well have deemed it too powerful and overzealously decided "why take the risk?" and sweep it up with the rest of the mods to be banned. I can see the concept that it could allow users to access locked content (that has to be paid for with virtual currency) and port models between games sparking concern, however unjustified the actions where. They also might have a different concept of what cheating actually is compared to the average forum member here.
If the senior decision maker, probably not the most technical adept in the room, decided it was too much of a risk, anyone else speaking in favour of Open IV will have been over-ruled, and that will be that.
The problem here is that anything even with the perceived potential to damage GTAO would feel like a threat to those lacking knowledge but not power in the corporation. The benefits to the franchise that Open IV has brought are potentially massive, but unfortunately, very, very difficult to quantify - otherwise it would have been easier for someone internally at R* to build a case to leave it well alone.
Just if anyone's in any doubt, I think it's a bad, misguided decision. But that's currently my best guess at what's happened here.