Colliers and supply ships are fleet auxiliaries with no purpose aside from supporting the military, so it makes sense they'd be classed as military. They also do not need to be particularly large, unlike freighters, sorium harvesters, etc.
Also, they don't give you ulcers like military-rules freighters would, since all supply ships do is follow fleets or occasionally make runs between colonies.
You say they don't need to be particularly large. On that note, I feel like I'm doing something wrong with colliers. Are magazines just massive? Is there an efficient way to cram a whole lot of 'em into a ship?
For anything that seems useful (not just constantly oscillating between colony and fleet) I keep ending up with ~40KT designs. It just seems very expensive to produce a glorified freighter at a naval shipyard. But I really just think I'm doing something wrong.
It seems I can design a reasonably useful hybrid collier/supply ship at about the 40KT scale. 2500 ASM (roughly 12 fighter wing reloads on top of the 5 reloads that the carrier will lug around) and 40000ish maintenance supply points. Is this a good way to go? Overkill?
I've tried a design with fleet speed/2, but the space freed up by dropping the engines was so quickly consumed by magazine space that it hardly seemed worth the compromise.
I still haven't produced an armed vessel in Aurora. Inching ever closer. What a process.