It's naive to say that this behavior is limited to Cozumel or to Mexico.
There are many websites devoted to finding mules for merchandise imports to other countries all around the globe and none mention declaring and paying duty at entry. In fact, some inform on how to beat the duty, especially for countries with high import taxes. Luckily, Mexico's taxes are not high.
How many US tourists return home without declaring their overages on cigarettes and liquor? Many, many, many.
Years ago, I toured China by bicycle with an international group of elite cyclists. For their return home, many in that group padded their bicycles (in bike boxes) with Chinese rugs -- I'm talking 4-6 silk rugs per person -- and, to my knowledge, no one declared those rugs or paid import duty to their home countries.
Same applies to the international movement of artwork, jewelry, designer clothing, electronics and other items.
I'm not posting about the specifics of regulation. Just saying that non-declared importation by individuals is a constant reality all over the world, which is why inspectors have work...