According to the official MLB draft rules, only residents of the US, Canada and US territories (including Puerto Rico) are eligible for the amateur draft. All foreign residents are eligible for signing through a number of independent processes.
According to one blogger, who cites Steve Perry from Citypages, this exclusion implicates a number of pitfalls for Latin American players:
1. During its' role out of PED testing, Latin American players were
testing positive at a 2-1 rate over the rest of the players. 2. Often
times, players are denied help. In addition to a language barrier, there are no
agents, players are hidden in academies and sold to clubs, money is given to
parents, and the average signing bonus is 3-5 thousand dollars. 3. These players
that are hidden in academies are often times, targeted at the ages of 14-15
years old. They are trained and sold. These "go betweens" get a finders fee + a percent of that signing bonus.
While an estimated one third of the players on MLB rosters in 2008 are foreign-born—the majority of whom hail from the Dominican Republic and Venezuela— this number could rise to nearly 50% in the next few years, according to the Sporting news.
Sources: Sporting News, Major League Baseball, NY Times