Upvote:3
You are correct, that it is the passenger's responsibility to check that they have all the rights to enter the countries for every layover. In particular, if you are not allowed to board, then Turkish Airlines will not be required to give any compensation.
The good news is, that the travel advisory does not prohibit people from transiting through Turkey. In fact, since practically all Europe -- US flights are cancelled due to the travel bans of both countries, this is the currently the cheapest way for me to transit between the two, given that I have the right to enter both. Get on a Turkish Airlines flight from Europe to Istanbul, and from there a Turkish Airlines flight from Istanbul to the US. Since Turkey is not on the travel ban of either country, the flights continue as planned. Also, since I am not "entering" Turkey but merely transiting through it, this is also allowed by Turkey.
p.s., I am aware that this particular flight had been cancelled; the above is general advice for others in the same situation.