Actual cutscenes dictate your character's emotes. Some of the idle poses would look pretty weird if combined with certain emotes or facial expressions. So even if they allowed non-default poses during neutral parts of the cutscenes, you'd be forced back to the default pose in others.

I agree that the default miqo'te male pose feels kinda tense. As much as I'd like to see more nuanced character control, it would probably be more realistic to adjust the default pose to be more relaxed than to rework cutscenes to allow non-default poses.