I think the engineering part comes in from the testing for a battery of things.
Fortunately since international companies want to sell all over the US and Europe, it has meant that this is the new baseline most of them shoot for. "Legal in California" means legal in the US (and likely EU).
It is better for everyone. It really is.
Off the top of my head, I've witnessed this label at parking garages and Starbucks drink counters. For public locations, if the label's concern were legitimate wouldn't it be appropriate to fix the situation instead of just fostering an environment of ignored signage and faux consent?
It also has the secondary effect of making REAL cancer warnings seem trivial. "Oh cigarettes are bad? So are electric cords and pepsi according to California!"
Absurd regulations (like putting worthless "product may cause cancer" labels on everything) have resource costs. These resources could be put to better use.
A lot of CA regulation is like the TSA. The cost per life saved (or harm avoided / prevented) is astronomical. There are far better uses for the resources squandered.
Except of course when they get it wrong and have to reverse course, as seen in the news over the last several years in the case of flame retardants for furniture.
So, my sense of the current "wisdom" is that the cancer and toxicity issues are significant, especially for firefighters, and the retardants offer relatively little protection against fires. A big factor in the turnaround is that the California standards were primarily concerned about protecting against dropped lit cigarettes, and so many fewer people smoke, particularly indoors.
http://newscenter.berkeley.edu/2012/11/15/pbdes-and-neurodev...
edit I didn't notice but zerocrates had already pointed this out.
Oh wait there wouldn't be one.