Duffey did not throw at Mercedes. That pitch went exactly where he intended it to go. (IMHO.)
Sixty years ago the first pitch in Mercedes' first plate appearance would have been in his ribs at 90 mph. Whether or not that is right and good, both players, both teams, both managers, and all four umpires would have accepted it as part of the game. And that would have been the end of it. Of course, sixty years ago no team would have had such little pride as to have a third-string catcher lob 45-mph tosses from the mound. And, of course, even if that had happened no batter would have swung at 3-0 in that situation.
It all comes down to sportsmanship. Do unto others as you would have them do unto you. Or something like that.