Bank robbery typically involves violence or the threat of violence. I don’t think the two are really comparable.
To justify imprisonment (rather than purely monetary penalties or civil judgements) you really have to look at the relative scale of harm and the type of harm inflicted. Imprisonment doesn’t always make sense, there are many nonviolent crimes where a purely monetary penalty would be more appropriate. For one thing, imprisonment can prevent people from paying back the victim.
Not necessarily. One can walk into a bank, slide a note and a bag, and (hopefully) walk out. No threats involved. Still a federal crime.
I feel like you are missing the rather obvious or else the note implies. Luckily the law is not so rigid as to be hamstrung by such sophistry. I didn't assault you, I merely invited your face to make an acquaintance with my fist!
The reason the note works is banks don't want to pay the cost of having their employees getting their brains blown out. Passing a note with this understanding means one knows they are making a threat.