I'd have to see the exact text of this law before I disagree with it.
But I must say I can understand it. Enough studies have proven the potential damage of second hand smoke that a case could be made for this.
I suspect that the legislators are doing this with our litgious society in mind. Lets say a 20 yr old gets diagnosed with lung cancer. He was placed with foster parents who smoked heavily at age 5. Now he sues the agency that placed him because they put him in harm's way.
I don't think that's far fetched.
I agree that smokers are not necessarily bad parents, but smoking in the presence of a child does seem to be outting them in harm's way.