True, but that's not what I was referring to. When adding a texture to a sprite collection, I see that it gets converted when the collection is committed and built, and that's perfectly fine. What I was noticing was that all my textures were being converted the moment I imported, and couldn't be changed back regardless of whether or not I intended them for use in a sprite collection. This meant, for instance, that small icon textures I was using for custom editors were all getting locked in Truecolor mode and adding unnecessary bloat to the project.
I believe I found the answer, though: in the SimpleUI files there's an AssetProcessor script that overrides the OnPreprocessTexture callback to automatically set the texture import settings. I can't credit why this is necessary, as once again not every texture in my project is meant for use in the final GUI. Commenting out the content of this script allows me to set my textures by hand again, without apparently harming the process in 2D Toolkit of converting them for proper use in sprite collections.
There's no SimpleUI forum that I can find, so I hope others that use that package in conjunction with 2D Toolkit find their way here to comment on the purpose of this script.