I just posted about this. Who's going to pay for their return ticket? Where will they be housed until then?
From what I've read and my own experience it's not the carrier's responsibility to check visa status before travel.
it is the passenger's responsibility to pay for the return flight when turned away at an airport (though their return flight would usually cover it) - if they can't pay then it is down to the carrier...
perhaps if that were the case for Ferries (should the border be moved back) then they'd be strongly incentivised to ensure return tickets had been booked when some national from a dodgy country tried to get on board

