If the DF engine had a customizable interface, other people would create interfaces that would completely hide the engine created by Toady from the player, which, in turn, would decimate the donations to Bay 12 Games.
Even though I recognize that it's not really
your argument, I find that argument extremely hard to swallow.
I find it very hard to believe that anyone could actually
disguise Dwarf Fortress in a way that would actually fool people into believing it was somehow a different game. Even when you're playing on Stonesense, which has an entirely different graphical interface far removed from DF, there's no way to avoid realizing that you're playing DF, or what sort of mechanics are going on under the hood.
As long as the only thing that is moddable is the graphic interface, you wouldn't even be able to get around the fact that all the rest of the game is still hard-coded and has that whole world generation sequence with the large customizable world parameter set in it that would be a dead giveaway.
Sure, you could try removing features, but then you're winding up with a vastly inferior game to a game that is already being given away, once again, for free. Oh, and they can't try to improve upon their "own version" of the game, since they don't have access to the underlying engine.
Now, while yes, someone could try decompiling DF wholesale, that's not something you're stopping by preventing anyone from modding the graphics.
Further, all you'd be doing is repackaging the same product Toady is already giving away for free... and how, exactly, does that cut into his donations? It would only work if new people joined up, wanted to donate, but didn't know that Toady ever existed because "evil program-disguising modder person" didn't tell them... and I seriously doubt many people would be eager enough to donate for a project without having the will to look at the people who would be screaming about how someone was trying to repackage a game already being given away for free.
Besides that, even if someone DID try to start selling a product that, again, is already being given away for free, the worst thing that would happen would be that it would raise the awareness of DF, and attract more people to DF in the first place... which is a
good thing.
The real competition for DF is not someone trying to "steal" DF, it's someone trying to build a
better game, and that's not going to happen by simply throwing a coat of paint over DF. And when people try, they often wind up only raising DF's profile more, as Minecraft wound up helping popularize DF, itself.
Ranting aside, I found that most of the birds in the new version of your graphics set have those birds set as regular-sized versions of the birds. To the best of my knowledge, most of those birds are vermin, and as such, they are not going to be visible as graphics, and those were meant to be the giant versions (and I'll go back and make the non-giant versions of creatures which aren't vermin later by simply resizing.)
Finally, the way that sand walls and the "~" type tiles looked so similar annoyed me, and I really greatly prefer the flux materials not have an icon, and have an old "+" I preferred more, so I went and did some edits to your tileset.
Maybe you don't want to see them, but maybe you could add some of these in as further alternates you can include in your tileset editor, so I'll just put it up here.