Reflecting back on my own experiences, I can say that knocking hasn't been an absolute office norm for me. By and large, if someone softly tapped on my open door to announce themselves, it was usually a co-worker or a subordinate. Supervisors typically walked right in. I don't startle anymore than the average person and never gave either approach much thought. Every office has their own norms that evolve from the culture of the business.
And more often than not, when a supervisor was coming into my office and immediately began to speak, it was due to the fact there was a pressing issue that needed to be addressed. Tabby, can you pull up the notes for the Smith case for me? I have a call with their attorney in 10 minutes.
Stuff like that. The intention wasn't to be rude, they just needed something. It's a business. A lot of interactions are need based. It wasn't a norm that every dialogue we had needed to be prefaced by a quick social nicety. Again, that doesn't mean we can be deliberately rude to one another, but the emphasis is on getting something done so we can move on to the other 100 fires needing to be put out.
Essentially, work is going to be a place where people have a lot running through their minds. I don't perceive the supervisor to be willfully unkind. I just figure as she approaches Mom21Son, her inner dialogue may be along the lines of, "Got to grab the budget numbers for the conference call at noon. Bob can't make the 3:30 meeting and I have to pick up the files from Jill. If we don't get approval on this contract, Big Boss may have to let some people go..."
If people remember to knock or make a bit of noise, that's fine. I'd do it if I knew I'd temporarily terrify someone otherwise. And I have a soft step too! Office ninja and all