It's due to how the past-game characters were implemented here. In the Iwata Asks about Awakening, they talked about how the DLC and Spotpass characters were a last minute idea, and you can kind of see it in things like that. The Bonus/Spotpass teams use basically the same engine that's used for Streetpass teams with very small tweaks. Even the number of lines they have (3, intro, battle and joining) is taken directly from the streetpass feature.
Anyway, after you buy a item from a streetpass team, they disappear, and, because the player can't call them at will, that's a significant problem. However, the spotpass teams can be called at any time, so that problem basically doesn't exist, and the team disappearing after selling one item becomes just an unintuitive interface. The whole set up is just there because they're reusing the same system they had originally built for streetpass.
I really like the fanservice though, and I hope we'll see another FE where they'll have the feature in mind since the beginning.
