It’s complicated. But I think I can sum it up, if you don’t ask too many questions:
Snap knows the license of the snap in the store. It doesn’t know the license of the snap you downloaded – there’s no guarantee they’re the same.
Add a license: GPL-2.0+ to your snap.yaml and everything will line up.
Unfortunately snapcraft doesn’t support it just now, so you’re probably stuck unless you were to patch your snapcraft, but that’s crazy talk.