Lior,
1.) As CharlieHall pointed out above barring entry comes at an extremely high cost.
2.) The point is that an ineffective quarantine (like barring entry from countries with cases) would in any case prove useless and only prevent people from actually fighting the virus. If it wouldn’t have come over here with a West African then it would have come over with someone who had contact with a West African, or someone who had contact with someone who had contact with one etc.
An ineffective quarantine is about the most harmful thing you can do in a situation like this