I'm convinced you haven't. Take a look at any of these crazy news videos where there is heavy gang violence or cartel violence happening openly in the street in some of these 3rd world South American countries. Stop being too idealistic and not watching or paying attention to the news that matters.
What ends up happening, especially in this girl's case of visiting Costa Rica, is that a very poor area allows or decides to build a resort on one of their scenic beaches. People looking to go on a tropical vacation see this in a magazine or an ad and its way cheaper than going to the Bahamas, St. Thomas, Barbados, etc. Meanwhile they haven't done their research. The area just recently was 3rd world conditions until they built the resort. The surrounding cities or rural areas are dominated by high crime related to gangs or cartels.
The teen boys in that area are usually heavily involved in crime. Not all of them, sure some of them might be just nice lads. But hanging around a tourist trap? When was the last time you, even as a teen, hung around the tourists willingly and their lack of knowledge in the local cultures or manners didn't get on your nerves and you didn't have ulterior motives like hooking up with some chicks who would like your accent?
At the very least those boys are trying to score some foreign poontang. And if they haven't been raised in the most forward thinking of societies they aren't going to be pleased when they are told no. Especially in a country as poor as Costa Rica was during the point in time HysteryMystery was talking about visiting it. You've got this combo of crippling poverty mixed with cartels and I doubt the teen populace have watched many PSAs about what is date rape and how just because a woman seems like she wants it, saying no means no. I doubt they've watched much Lifetime or paid attention to tumblr or youtube. Chances are, as surprising this might be to you, some of those kids still don't even have internet in their homes.
Sure, you can't assume everyone is evil. But think rationally. Pay attention to the history of a place. If that place has a bunch of poverty outside of its tourist locales it means that the locals aren't people you want to mingle with as they aren't looking just to have a pint and some conversations about football. You can't assume everyone is evil, but as a 14-17 year old girl its sure as fuck better to be safe than sorry.