Why do diabetic patients have abnormally acidic urine?

NOT necessarily. A normal urine pH is 4.6-8.0. (remember 7.0 is neutral &<7=acid). A low urine pH (acidic urine) may be due to abnormally high blood sugars causing diabetic ketoacidosis (not the usual norm). Other causes of 'acidic urine' are: diarrhea & starvation. Some medications lower urine pH e.g. Thiazide diuretics. A diet high in meat products or cranberries can decrease your urine pH (to be more acidic).
Possible. Usually, urine is acidic. In cases where the blood carries a higher than usual acid load, it is natural to have that acid passed on to the urine. Diabetics are prone to metabolic acidosis, in particular type 1.