I think you are applying the wrong logic on this.
The limitations like Backface culling and the minimum object dimensions only apply to TMTK, it is a converter that was programmed to, well, convert 3D models for the ingame shading and rendering,
the devs don't have this "simplified" TMTK, and Backface Culling could be added as an option, but as far as i can see, they tried to keep the converter as simple and as stable as possible.
After all, not everyone who does custom content knows a lot about how to make a performance-effective asset. This includes when to use BC and when not.
Some of the vanilla content uses backface culling, most of the opacity/alphamap objects do anyways. If that wasn't the case they wouldn't use alphaplanes, because the whole point of alphamapping is that you save on polygons, and only a terribly programmed game would not make use of two-sided polys. However, this doesn't concern the TMTK, even though it's a wrong assumption to keep this option from the ugc folk, because i, for one, could work more performance friendly when it comes down to plants or LOD replacements (3D >to> alphaclip), but without two-sided polys i wouldn't save many tris ecause i'd need to use double the count than if i could uncheck BC.
The minimum thickness, like the max. size, are just limitations they put in for us.
Why would they need this? An asset artist makes asset A, passes it on to his/her supervisor who is in charge for implementation and who decides what goes in and what doesn't.
Basically, long point short, the TMTK is streamlined for ease-of-use. Don't assume this is the same tool that a dev has available,
it might build upon it, but it's very bareboned, probably out of fear it would overcomplicate things and cost more development time (and money).
It's like you have a VHS and they have a YouTube streaming service, and then they offer a conversion tool where you can upload the VHS to the streaming site.
They might have Blu-rays, but since you have a VHS they added a tool that allows for conversion. That doesn't mean they need to, or would, use the same tool.