I guess the concept of "any eligible roll is allowed, unless against the run's rules" can really be too complicated and incomprehensible for some, to the point of making them only join guild raids, sure.
As the rule covering this states, GMs are not meant to be a shield against ninja, to the point players can just go on cruise control and then complain when something doesn't go their preferred way. They are strict and simplified, probably for the sake of GMs not having to analyze and make special judgement on each and every case, balancing all sorts of claims of "this isn't good for X," "that is better for Y" or even "Z barely helped and doesn't deserve it."