Ok, if you look at individual actions, burning the plastic makes complete sense. But if you look at the economy as an evolving system, you'll discover that if you do nothing together with burning the plastic, society will just adapt to consume the extra oil, while if you bury it, people will probably not make an effort to recover that plastic any soon.

Of course, you can burn the plastic and make some change that makes people burn less oil. But if we are talking about government intervention, you will get much more return if you focus on reducing the fossil fuels usage alone and bury the plastic.