Re: Companies probably aren't doing this to discriminate
They can't remove the cost difference, they can only remove the option, because the cost difference is merely because Facebook charges by the reach (i.e. number of people who see an ad) which is why it is in the best interests of an employer to maximize the percentage of people who see the job posting that might be interested in it.
One could argue that advertising an oil industry job in Men's Health, or a nursing job in Cosmo would be almost equally discriminatory given the gender disparity in the readership of each. Heck, just advertising on the internet could be seen as age discrimination, given that the older people are the less time they spend on the internet.