This is how Sweden finances their free education as well, it is called taxes. The problem isn't with how Cuba treats specifically doctors but how Cuba refuses to let people emigrate in general.
Note that this is a problem not limited to communist countries like Cuba. In America, in particular, we have this problem when it comes to our teachers. Teaching the next generation is one of the most important jobs we have and, yet, the pay is so low that the best people for the job are disincentivized to go into that profession. When a hedge fund manager managing teacher pensions out-earns an entire school faculty who are invested in his fund while underperforming the S&P, we too have a problem.
Presumably these doctors don't have that choice; they get what the government says they get and that's that.
Of course med school in the US is far from free. Totally different system over there, if you compare a single part at a time of course one of them is going to look terrible.
The Cuban people should have more freedom of choice and freedom of movement, but economically, these kinds of arrangements present themselves under capitalism as well.