There's a few things to consider.
Firstly, "unmoddable" engines have long, long ago gone extinct. There's just no such thing, every single engine out there is designed with easy moddability in mind. The reason for this is that games are no longer made by programmers and graphics artists (they haven't been for nearly a decade now) - they're made by game designers, as well. Game designers are people who don't necessarily know how to write a real program - instead, they rely on tools supplied by the programmers to do their work. These tools aren't a part of the engine as such, they're external - so obviously, the game data has to be stored in such a way that it can be easily accessed, read, and modified with an external program.
Secondly - an engine being moddable doesn't mean that the company releasing a game will help people mod it. Take WCP for example - there has never been a more moddable Wing Commander game. It's absolutely fantastic, you can do virtually anything with it - but Origin and EA were able to prevent easy modding simply by not releasing any of their tools. In other cases, companies release specially-designed, cut-down versions of their own development tools so that they can control what people modify. Of course, none of this ever works - at the end of the day, with every game in the world being moddable, the only thing that prevents games from being modded is their unpopularity and/or short attention span. That is to say, if you play a game, finish it, and have no reason to play it again, you're not going to bother modding it, either. The only games worth modding are those that you come back to again and again.
Finally, about the reputation-spoiler argument. It's a pretty darned crazy argument, and in a logical world, it would be thrown out the window right away. After all, the GTA mod just revealed existing content, and the various other "embarrasing" mods (for example, there's a lot of games out there, from Tomb Raider to The Sims, that have nudity mods)... well, nobody ever cares about them except the half a dozen teenagers that waste their time creating mods to strip virtual women of their clothes. However, these days everybody is constantly worried about getting sued, and it is apparently preferable to be insanely over-cautious than to follow common sense. It's also preferable to be over-cautious for another, more important reason - the more talk there is about bad mods, adult mods et cetera, the more likely you are to have congressmen and senators calling for new regulations and the like. So, in that aspect at least, it really is preferable to be cautious - it's just that in many cases caution is taken to the utter extreme.