Only thing I can come up with is the fact that lets be honest, a lot of truck drivers don't have the best eating habits. Not speaking for all but most.
One thing that will get you kicked out of a companies physical real quick is high blood pressure. Most foods can make your blood pressure rise quite a bit.
This actually happened to me once where I took a blood pressure test and the doc told me to eat right, sleep good, and don't smoke the next morning and my blood pressure dropped dramatically. Also in some cases at your physical the doctor can make your expiration date on your medical card whatever he feels necessary.
If your bp is high if he passes you it may only be valid for 6 months and if you dont drop it at that point he will pull your card til you seek medical attention. I think they can only go as far as 2 years in which you really want so you dont have to jack with it.
So to answer the question I think they say dont eat to keep your bp down.