Well, after spending 10 mins on beta, I've now experienced it for myself.
I can imagine the devs discussing this at a meeting.
Dev A: People are saying monster aggro is too annoying. We should probably fix that. Any ideas?
Dev B: How about we make all monsters aggressive, instead of only some of them? And then make it so that players can't even go to the toilet safely without going into their haven bag first?
Dev C: We don't want to annoy the high-levels when in areas they barely ever walk through anyway, so maybe we should make lower level monsters not attack higher levels.
Dev D: We don't want to go that far. At least some monsters should always attack high levels no matter what.
Dev A: Well, this sounds good to me. Let's get to work!