Perhaps it is more nuanced than that: getting the patient to eat after surgery is probably the most important thing - and perhaps burgers are the way to do that for the majority of patients?

A quick Google of keywords (weight loss after cardiac surgery) shows papers with sentences like "Patients who lost weight faced a significantly increased risk of mortality than those who experienced no changes or gained weight after surgery", and "We conclude that obesity confers a survival advantage in the setting of the CABG surgery", "While various large-scale randomized controlled studies evaluated different post-operative nutrition strategies in rather mixed cohorts of critically ill patients, only few small clinical studies specifically investigated its effects in cardiac surgery patients. In these studies, malnutrition has been reported to increase morbidity and mortality after cardiac surgery".

I agree it is obvious that good food is best, but or hard to know what the actually constraints are. I think it is unlikely the NHS will put on a nice entrecôte de boeuf!