Because there was a law passed in the United States in 1978 called "The Pregnanc...

guyzero on April 7, 2015 | parent | context | favorite | on: Silicon Valley: Perks for Some Workers, Struggles ...

Because there was a law passed in the United States in 1978 called "The Pregnancy Discrimination Act" which makes it illegal to fire people based on their pregnancy status. So assuming the commenter is in the US, it's the law.

http://www.eeoc.gov/laws/statutes/pregnancy.cfm