A common posture, to be observed by all participants, is a sign of the unity of the members of the Christian community gathered for the Sacred Liturgy: it both expresses and fosters the intention and spiritual attitude of the participants.
But, yes, you are correct that the posture is of secondary importance to the 'intention and spiritual attitude'. They are putting the cart before the horse in many cases and would be better advised to improve their catechesis.
It was sort of humorous at RCIA when the question regarding genuflection before receiving Communion came up. The director would have liked to have said 'forbidden' but she didn't. She was searching for the right phrase and I suggested 'not preferred' and that was what she used.
I was pleasantly surprised to hear my pastor give a little talk before one of the First Communion Masses to all the non-Catholics in attendance about not being able to give them Communion. He was diplomatic yet forceful.