The U.S. does have paid maternal leave. In fact, I had 6 weeks paternal leave at my old job. The benefit was available to all employees at the (Fortune 500) company.
Apparently however, I picked the wrong career and should have been a coach. And people wonder why the US is going in the shitter. Must be all that socialisms.
http://www.jec.senate.gov/archive/Documents/Reports/03.05.08...
Edit: It also notes that among Fortune 100 companies the median maternal leave is 6 weeks.
By mandating it, an employer has to consider the possibility whenever they hire someone, and so salaries will be lower as a result. If the employee doesn't get pregnant and use the leave, their salary has been kept lower with no benefit to them.
We have discrimination laws against that.