You honestly have me with that question. My husband wears shorts about 11+ months out or the year here in Florida and I can only remember 2 times he wore them in the DR.
One time it was so cold he even regretted it. His attire now consists of 2 pairs of jeans (1 black for formal night 1 blue for casual) and various shorts.
I personally couldn't care less what people wear since the quality of the food is thee most important thing that concerns me. If people like shorts for dinner, so be it. It works for breakfast and lunch, so why not dinner?