It's not ignorance, it's common business sense. Suppose you couldn't work full-time because of reason X. It doesn't matter what reason X is, I can not use your work in the same way, if for no other reason than Brooks's law applying.
Unless you're an anti-natalist, you can argue that X=pregnancy is really important to society as a whole, but then the burden should be carried by the whole of society, not just the slice that happens to run a business.
Either way, we have to admit that you can't be a mother and have equal career prospects to someone who stays childless, just like you can't put the same amount of work into two different careers and still get the same result for each as if you had put all the work into just one.
Unless you're an anti-natalist, you can argue that X=pregnancy is really important to society as a whole, but then the burden should be carried by the whole of society, not just the slice that happens to run a business.
Either way, we have to admit that you can't be a mother and have equal career prospects to someone who stays childless, just like you can't put the same amount of work into two different careers and still get the same result for each as if you had put all the work into just one.