On our recent Princess cruise, they offered freshly popped popcorn for free every night before the "Movie Under the Stars" (movies shown on the their "Funnel Vision"). It's now one of the few things that actually angers me about DCL that they charge $3.50 for a bag of popcorn. There's just no reason for it.
Yeah, this does seem odd. It might be because they are pre packaged or they maybe are just trying to do some inventory control or don't want everyone to have a bag of popcorn to make cleanup easier? Who knows?