While free thing can be nice, they have a propensity to be overused and eventually abused.
Research shows that giving away low grade plastic bags for free has a direct impact on the number that end up blowing around our roads, schools and rivers. You could say this is a litter problem, or you could say it is a problem of people not valuing them appropriately and not taking care of them (kinda the same thing really).
We think that if something is having an impact on how we all live then that thing should be priced to include the collection and disposal at the point of purchase. Otherwise those who choose to do the right thing are penalised the same as those who do not.