I think part of the "swinging at a 3-0 pitch" has to do with the situation at the time. If the game is tied or you're behind, no one is going to say a word about swinging at a 3-0 pitch. but when your team is ahead 15-3, and a position player is lobbing the ball over, swinging at a 3-0 pitch is seen as 'rubbing it in' according to the unwritten rules.
no different than a guy trying to steal a base in the 8th inning with a 7-run lead. It may not be against the official rules, but it's not "how the game is supposed to be played."
Or, let's say, a basketball team with a 30-point lead and still using the full-court press. or a football team with a big lead that keeps its starters in the game in the 4th quarter.