Honestly, that's got to be the most snobbish, stuck-up community standard I've ever heard of. Granted, it's usually best to go ahead and seek permissions before using other people's work. However, by the simple facts that 1) these meshes are not the original work of their creators but rather almost certainly unpermissioned copies of someone elses designs, and 2) the fact that these models are available without hacking their computers is generally more than enough grounds to assume that they are for public use. All the FS:AAB mod did was convert them from homeworld meshes, right? I don't see how "ripping" them is grounds for a **** off letter, or such a defensive "our content, our content" whine. Really now people, grow up. That's not really any different than converting a model from Homeworld, with the sole exception that these models are not credited to the homeworld staff so crediting them properly takes research. Unless of course they were somehow planning to make money on the deal, in which case I say go right ahead and see how long it takes both Sierra and whoever produced S:AAB to sue their asses off. They are right, this is a modding community. By that fact we assume that we all would rather see our work reach as broad an audience as possible, and should take the use of our work by others (so long as it is properly credited) as a both a form of flattery and of publicity for the rest of your work. If they want to be dicks about it and deny permission now, then that's their problem, but they would never have given the permissions to begin with from the responses they are giving here and in their own forums. For that matter, since they are a part of a public mod, don't they practically lose claim to the models anyway? Unless they went to the trouble of making them legally theirs by copyrighting them or whatever (which they couldn't do, since they were taking copyrighted material and putting it into someone elses copyrighted file format) then unless I'm mistaken there's not much of legal or a logical claim that they can't be used.
To all members of the HLP community, please never act like such a child when someone else wants to use your work. It's embarrasing.
EDIT: And for the record, I would think that having "content" (in this case models) used with an appropriate link to the original project would be far preferable to not having them used; the reasoning that "you've screwed up so you can't have permission" is extremely childish. There's really no way around it.