Horses are generally treated as beasts of burden in places like Mexico & or any third-world country. Unlike horses in the US, Canada, etc, they aren't pets & have to earn their keep. Many people in Mexico (especially certain areas) are very poor & so the animals are fed just enough to keep them working.
Many of the horses are used to pack tourists around & the people who own them & care for them, again, are usually poor & use the horses for their income.
There was one area in Puerto Vallarta where there was a big string of these horses who spent all day carrying people up & down a mountain to one of the resort's big attractions. These horses had saddles on them ALL day, many weren't even tied up, just wandering around the area & when a tourist wanted to ride up instead of walk, the guys who owned the animals would grab a horse & escort the "riders" up the mountain.
I also love how on the internet & brochures for certai Mexican resorts, the horses in the pictures are all big, fat & glossy & when you get their they're these scruffy, skinny, 14hh things.
It is sad, but i suppose that's how it is in places like that, though i know there are other, more richer areas other than the ones you see near the resorts that take good care of their horses. Horse racing is a big thing in Mexico as well as Paint breeding.