We may be confusing the B&G right to have the wedding they want with the guests right to wear what they want within the dictates of the formality of the event.
The B&G are within their bounds to have the wedding they want with regards to venue, food, guest list, time and place, formality and whether children are included or not. The B&G are not within bounds to dictate exactly what their guests will wear. Dictating only "flats" is the same as dictating that no one may wear a dress shorter than 1 inch above the knee. The formality of the event may be dictated, not the actual clothes the guests wear.
We have become a society that encourages B&G's to "have the day they want" and "it's their Special Day!" to the point that the bridal party and guests wishes are not only completely disregarded, but the guests are seen as "rude" if they don't toe the line.
*Disclaimer* I cant' wear anything over a 1.5 inch heel, so usually wear "flats" anyway. However at 5'8" I am already over average height. Would the bridal party expect me to slouch all night?