Just as a side note -- I'd wait for more information on this before we call medical tourism in Mexico dangerous. Seems odd Mexican cartels would target random U.S. citizens who were on their way for a cosmetic procedure. It's possible they were complicit in nefarious activities.
I don't know a whole lot about how the Mexican cartels operate (aside from their extreme brutality), but I'd have to assume that killing random tourists from the US would bring unwanted attention to them from law enforcement, and as bold as they are that's something they wouldn't do. Nor would they really have a reason to kill random tourists who pose no threat to them whatsoever.
By the way OP, I don't follow the news and thus don't know anything about what happened. Perhaps linking to a source to the incident you're referencing would help ignoramusses like me (derp) understand things better without having to google-ize it.
And as to your question, the only thing I've heard is that folks from the US go there who want very odd surgeries that American doctors won't perform because of ethical or other reasons. I don't know how common it is. I think way more Americans go there for the drugs that are available over the counter. Viva la Mexican pharmacies!