In either case, it's like "yes, you have my attention. I cannot focus on anything at all until you hit enter, and I also can't reply because I have nothing to reply to".
I also can't complain like "hey if you're going to interrupt me can you just say the whole message so I can actually reply instead of being held in absolute suspense for many seconds of my life?" because that would be, frankly, even worse than what they're doing, which was done out of a kind of ignorance of etiquette.