Bringing a motorcycle or not to Mexico?

You're quite right to question the law, part. It's a good excuse to stop anybody, as happend to a couple I know. After pulling them over, the police followed them out to the beach where they had their helmets, likely trying to make them contribute in some monetary way. But in doing so they got their car stuck in the sand, and were quite greatful when the helmet-less riders pulled their vehicle free with their own truck. They wore their helmets afterwards. Of course, it's a pretty good idea to wear them if cherishing the health you have.