So I'll preface this answer with that as a former dealer I've always applied the rules and I've never really cared too much for what the players believe is the rules, or what is good etiquette around when cards get mucked.
Let me first just post the WSOP rule from 2019, which while not universal across all tournament spaces it's often referenced and or inspires other rule sets adopted by many tournaments, which I think is actually extremely fair when it comes to the above situations for mucking hands. Importantly this rule also clarifies you're entitlements too.
- Showdown: During a showdown where no Participants are all-in and if cards are not spontaneously tabled, the Floor People may enforce an order of show. The last aggressive Participanton the final betting round (final street where betting is possible) must table first. If there was no bet on the final betting round, then the Participant who would be first to act in a betting round must table first (i.e. first seat left of the button in flop games, high hand showing in stud, low hand showing in razz, etc.) Participants not still in possession of their cards at showdown, or who have mucked face down without tabling their cards;lose any rights or privileges they may have to ask to see any hand. The winning hand must be shown to claim the pot unless there are no other live hands at which point the pot can be awarded to the only live hand remaining. If a Participant refuses to show their hand and intentionally mucks his or her hand, the Participant in violation will receive a penalty, in accordance with Rules 40, 113, and 114.
The above rule really eliminates and clarifies the expectations of players during showdown situations, especially where a participant refuses to show their hand, i.e. they're penalised.
So in terms of why it's considered bad etiquette, this is my opinion, is because you're high-lighting someone's mistakes. The reason why I say this is I often believe it's either one of four reasons why a player doesn't want to show;
- The player is perhaps embarrassed their bluff didn't work and that they don't want to show.
- It's essentially a small angle shoot, i.e. they don't give you the information that A) You paid for, and B) you're entitled too.
- The player is saying ok you win let's keep playing, perhaps they don't know they should show and they just being honest ok you got it let's get on with the game.
- Perhaps in this particular case you aren't entitled to see what the played had, the above rule clarifies that.
I'm sure most players throughout their careers playing have been in both situations at one point in time, that it has essentially become an unwritten rule, i.e. etiquette, to just tap the table and let the player get away with not exposing their hand. In a sense I believe it's bad etiquette as people often become defensive when they've done something silly and if they can hide their mistakes they will do so, and by you not allowing it, thus people attach the dreaded 'bad etiquette' phrase to it.
Exactly as you said, as it is completely reasonable, I have no issues if a player requests to see my hand when I'm playing, in fact I will just show it during showdown if I was the last aggressor and or the first to act in the betting round when checked down, i.e. I always apply the rule above.
I believe it is considered bad etiquette as people become defensive of their mistakes and you're essentially forcing them to expose that mistake even though they've already said you win. Or in a phrase 'let sleeping dogs lie'.