Because they are protected by the Migratory Bird Act, states must establish seasons and bag limits for them. Many states have crow seasons (not sure about ravens), just like they have duck, goose, and dove seasons (also birds protected by the MBA). I was once told by the G&F here in NM that I couldn't shoot crows because NM failed to establish a season for them when the Act was passed. It's not that they don't want them shot (specifically), there is simply no season for them. Perhaps just an oversight that some states have never been motivated to correct.
As far as I know, there are a variety of federal laws that protect all birds (to some degree) except for pigeons, starling, and common (English) sparrows. These are the only three birds that can be shot without regards to seasons, licenses, bag limits, etc. For those of you more "in the know," please feel free to correct this assertion if I'm off base.