Morality as a metric
When games introduce a moral or ethical choice they're asking something of the player. Do you save the protagonist's partner at the cost of dozens of other lives? Do you do sacrifice that NPC to gain a new ability? Is it really justifiable to kill that puppy to progress the plot more quickly?
The framing of these scenarios mirrors the real world: are you willing to act unjustly, unethically, immorally, for some other payoff? Will you slow the pace of the game, or miss out on exciting content, to do the 'right' thing? If this is true then the choice becomes a subjective metric of the player's ethics, how good or moral the player is.
We can't take that framing as a given because of the wildly different contexts individual players are introduced to and invested in these scenarios. Killing an innocent person in a GTA game is lightheartedly trivial compared to forcing your 'Lawful Good' character to steal from child in a months-long D&D campaign.
Moral choices become a measure of the player's investment in roleplaying and how much they care about being an active participant in this fictional world. The scenario is secondary to the primary question of which actions provide the player with the best experience to them. The avid roleplayer might be willing to take a longer, slower route through a level to avoid killing an innocent guard that another player would dispatch, but neither is answering the ethical question. They both act in their interests, where the rushed killer experiences the next in-game content more quickly whilst the roleplayer maximises their experience by remaining invested in the world of the game. The metric a moral system measures is: how much of a roleplayer are you?