I would say they are worried about car pollution. Its the reason for all the computer controlled stuff on a car gas engine; or the use of DEF with a diesel engine.
Just like creosote they weighed the benefit against the risk. We can not do with out cars or trucks or planes until we come up with something better. So they tried minimizing the pollution end.
With creosote there is no way to reduce the pollution end and the benefit is greater than just doing away with it totaly. So they just regulate who can use it now to try and cut the risk to the public down.
Creosote is just like many of the spray chemicals farmers use. The benefit out weighs the risk for many of these chemicals but on the other hand you do not want every home owner to have access to them. They are safe when rules are followed but it is just eaiser to limit who can buy said chemicals than to train every soul on earth how the use them.