I don't know about Cuba but I do know that, for a long time, it was very desirable to get into medicine in India, Sri Lanka etc. to get a job in a developed economy. I remember reading a report about a similar problem in many African countries.
"Brain Drain" seems to be a real problem and I bet you the bulk of them are looking to earn a fat wedge (compared to their home country). I've seen some go back again, to be fair - but that's usually when the gravy has dried up and/or to "retire" to an easier professional pace.
Fortunately Cuba has no shortage - maybe the Cubans really are more motivated by helping people (or maybe it's just harder to get an exit visa?)
Meh...I admit I'm cynical. I don't deny altruism exists, I'm just very sorry to say that it's probably far below where we'd like it to be.