Whilst I think that firing a women for not wearing make up is ridiculous and stupid, I believe a company/employer should be allowed to fire her if it is in the rules/contract. The women would have to have known that she must wear make up for it to be fair though. I think it would be wrong to fire her without her knowing that she HAD to wear to make up. I also think it would be decent to give a warning before firing, however, it should be up to the company/employer as it's their business.
It depends on the job, in all honesty. If you work as a mime or Disney Princess, or an employee at a makeup store, than it is relevant to the Jon to wear makeup, and if you were not planning on wearing any than you should not have applied in the first place. But yes, in these cases it does seem logical to be able to fire women who do not wear makeup to work.
If a women doesn't where makeup to work, like if she's a waitress (I'm not saying doctor because if she wears makeup that might be in her way) and she did't want to where makeup then who cares. If you do fire her for not warning makeup, well I'm pretty sure you can't do that. But for a non-makup-requiring-job a women should NEVER be fired for not wearing makeup.
Although wearing makeup depends on what job you take (if you're a model, perhaps you should read your contract), I would condemn any job, especially corporate, that requires its female employees to wear makeup if they want to keep it. And perhaps it's time that even our runways and makeup stores need to pick up and praise minimalists- I mean, why not? They would be endorsing strong, real women who aren't afraid to be themselves as a whole, and not just showcasing an anonymous painted face any longer.
As a woman, I'd look down on any job that put my appearance above my skill.
If someone isn't wearing makeup in a job like a mime, Disney Princess, etc. then they're neglecting to do their job, which is to be in character. So if they don't look "in character" without makeup, they're not really being fired for not wearing makeup they're being fired for neglecting their job. I feel like the question isn't really talking about these jobs, though. And even in the case of a makeup artist, one does not have to wear makeup to work to be good at putting it on someone else.