@Nongnuch *******
well I'm telling him, and you, by the looks, that he is NOT right. An airline does NOT have the right to refuse boarding at their discretion. Immigrations has that right, airlines do not. Please stop spreading/supporting false information you both. Airlines can (and should) require fulfilling a country's officially documented entry requirements, but if something is not part of that, then they cannot require it at their discretion.
Would also become a mess if it would be allowed. Have a plane that fits 800 passengers. Sell 5000 tickets. Then just refuse 4200 passengers based "on your discretion". Easy money! 💰