"There are strict rules of etiquette."

Angel was never invited into Serenity, so now River-in-Angel's-body is similarly restricted. Which sucks, in River's expert (and terribly frustrated) opinion of things. Not that she wants Angel to have free run of her home; that would be bad if/when the aliens decide to bring Angelus back out.

She just doesn't want to stand outside the cargo bay, awkward and bowlegged, when she's got a bunk right there.