If you switch to your "civilian" outfit while in a store, it will highlight all non-civilian equipment (such as (most) armour and weapons) in purple.
Grain, meat, grapes etc. count as civilian items, but not butter. Why is this, and does it have any effect on gameplay?
2 Answers
As far as we now the only current gameplay effect of civilian items is that those are allowed to be worn in town.
As trade goods can't be worn it would seem there are no effect there. Although there may be some effect on economy or one is planned in the future. Something along the lines of different tax on civilian/military goods or garrisons favoring military food more than the general population.
But for now we don't know if there's any affect.
As to why it is it's hard to say for certain, but considering it happens to be butter it seem likely it's a reference to the "butterlord" meme. A description of the origin of it can be found here.
It may also be a random mistake, or intentional for other reasons. We simply don't know.
I didn't find any source validating my claim, but there is an ongoing meme/easter egg with butter in the mount&blade scene. It's likely an easter egg deliberately put there from the developers to further play on that joke.
I'm afraid I can't answer if this has any effects on the gameplay since I didn't have the time yet to play myself.
You can search for 'mount and blade: butterlord' in your favourite search engine and will find many more results explaining the joke and its origins.
