I've flown with companies that have pre-warned crew about requiring a doctors note if they were sick over Xmas. I never really thought about the legalities of it. My guess is that if the unions agreed to it, then it must be legal. If you don't have a union, then you are on your own if you wanted to challenge the legalities.
If you are genuinely sick, then really you should have nothing to worry about. This rule is also for the benefit of colleagues who are on standby over Xmas as well as the airline protecting its operation. Perhaps thats why unions haven't opposed it.