Is this a contradiction or what? If people are choosing low carb options from the standard menu, guess what the dieticians will be making for them? A low carb meal!
Now, if they remove all low carb options from the menu, there might well be a problem (and I can envision them doing this just to enforce the "no low carb" policy), I can see this being challenged since what they are saying is that the patient does not have a right to chose what foods they will and won't eat and it does not cause any extra work to include low carb fruits and veggies on the menu and let the patient choose what they will.
I can also see this becoming a discrimination issue. Do they force Kosher people to eat non-kosher? How about vegetarians..do they force them to eat meat or do they accomodate the special dietery needs/preferences of one group while denying them to others?
OTOH, there's also the option of simply chosing a different hospital if possible. I know I would.