.. My opinion on the IMG and their ships is that they'd probably sell it to anyone with atleast a neutral/good reputation. They're a Corporation above all else, everything they do, they do in the hopes of obtaining more wealth. Honestly, I would not be surprised at all if the IMG sold their Gunboat to nearly anyone, even pirates, under the condition that said pirates wouldn't shoot them anymore.
As far as =CR= is concerned, and I don't mean to offend anyone here with this, I never have, and likely never will consider them to be the "official" IMG faction. Why, you ask? Because they don't RP as such, they RP as the Colonial Fleet, which has little to do with actual mining. Just because Galactica had to do mining operations sometimes in the show out of necessity doesn't mean that they are a "miner society/organization." They only mined because they had to do it in order to survive. The only reason they had to mine is because there weren't Corporations around to do it for them, anymore. Just because you patrol systems that are lucrative mining regions, that isn't to say that you're a Miner. I've never seen any of them actually mine or act like miners, but I don't see much of them to begin with, honestly.
.. Moreover, it is the Independent Mining Guild. From my perspective, and I've thought about this alot since buying my own IMG Gunboat, it is exactly as it sounds. It's a guild of independent miners who don't really adhere to any real corporate governing structure, but instead are obligated to work together in order to turn a profit. They're comparable to Junkers, from where I'm sitting; if they could get away with selling their ships for top-dollar to factions and gain immunity to attacks along the way, why wouldn't they? A good portion of the IMG's employees/members are probably freelancers themselves, honestly. Most IMG miners probably only do mining work when they need some extra cash, and the IMG as a faction is probably one of those groups that doesn't ask questions when you fill out your application to mine for them. They'd probably hire just about anyone to haul rocks for them, and in doing so are likely able to turn even more profit by having the ability to pay/offer whatever they want with their employment contracts. Think freelance/mercenary mining - that's how I tend to percieve most IMG members.
Until the infocard says IMG ships are only used by IMG enforcers, I'll continue to treat them as being freelance/civilian ships. The IMG shouldn't be compared to the Zoners, either, as they're very different groups. Zoners are more of a "civilization/society" than they are a "faction", and so they'll always have different rules than other groups. A faction is a gathering of individuals with similiar interests, and Zoners don't fit that category at all. Zoners as a concept are more along the lines of being a "way of living." Other groups are different, usually what holds them together is a belief or an idea, whereas Zoners just want to do their own thing - they have their own culture, which just so happens to have alot to do with profiteering/business. They're not a Corporation like most other business-oriented factions. Talking about Zoners is seriously comparable to talking about a House faction - they have bases, planets, a fully-fledged shipline and an extensive zone of interest, and so on. It's hard to compare something like that to any 'small' faction.
Edit: Besides, there's no real problem with non-IMG people flying that GB. Most of the people who're flying it that aren't IMG aren't even using it for PvP, but instead tend to use it as a PvP-deterrant. Limiting that GB to only the IMG would just result it in never being used, since there's so few IMG players to begin with. The CR when you see them are usually always flying Vipers, the only people I've seen use the IMG Gunboat was the [IMG] faction, which I don't see at all anymore. It's a good-looking ship that not many people are using, and those who do in my experiences are pretty responsible people, please don't restrict it pointlessly.