All the boss has to say is 'she told me to 'f' off, so I fired her' (whether it was true or not) and he gets around the law.
IMO, making a law that makes it illegal to fire someone strictly because they are pregnant would open up a whole can of worms.
And as far as I am concerned, there are too many laws as is.
Besides, all the woman has to do is tell the press that she was fired and that she is a pregnant woman who desperately needs her job. No boss that does not want terrible publicity is going to fire a pregnant woman unless he/she has one heck of a good reason. Just let market forces give this woman job security...not yet another law.
And finally, I think the government is involved in our lives FAR too much as it is - telling private businesses who they can and cannot fire is, IMO, none of the government's business.
But, each to their own.