Originally Posted by JackandJill:
I hate everything to do with smoking and yes while I agree we can't always avoid it, as an adult we can do our best to minimize exposure it. You can ask for a new desk away from a smoking coworker, you can move seats on the bus or get off or move out of line. I have let a daycare family go because their personal belongs and dcg clothes had so much third hand residue on them. I chose to make myself less susceptible to exposure. I have a strong no smoking policy, that includes exclusion from the daycare home if a parent had a cigarette in the car on the way to pick up.
A child simply can not do this, though, and their little bodies are taking in all these toxins while they are still developing. If your guest has a cigarette, and then comes back inside, days later that 3rd hand residue is still in your home. If it is near the rugs and furniture they touch, climb on and chew, they are completely exposed to those insane chemicals.
I agree that sometimes it feels like government over reach but I agree with Black Cat, there are other lines of work, or ways to legally operate a child care without a license.
Its kind of like when we tell parents asking how to get out of their contract. We all know the rules a regs before getting licensed. Can't be upset if you knew it all ahead of time. 
Sure, you can ask for a new desk, but there may not be one available that isn't next to a smoker. Tobacco is a big allergen for me. So are cats. It got to the point that there was no desk that wasn't next to either a smoker or cat owner. I just had to suck it up (or, rather, wheeze and break out in hives).
Sometimes getting off a bus or moving out of a line has serious consequences. What if that's the last bus that can get you to work on time (and your boss likes to fire people). What if the line up is at the dmv and you need a driver's license for work. Other times it's just massively inconvenient to get off that bus or out of that line.
Sure, there's a choice but it's not always a good or realistic choice.