The difference is that aluminum can be reformed into different products fairly easy via recycling. In addition, recycling aluminum takes less energy than refining aluminum ore. It's worth recycling. Plastic cannot do the above. It can at best be changed into an inferior product at high costs. The chemical characteristics are very different between aluminum and plastics.
Considering the environmental damage, why not use the tried and true REPUBLICAN method of market based solution. MAKE IT COST, to throw it away. Thus, making it like Aluminum.