To which I say, "So what? Why is that bad? Don't you folks want to lock up that nasty carbon? If plastic breaks down, it releases that nasty carbon."
Therefore, every plastic bag put in a landfill should be seen as a good thing by them.
Back in the 70s, the enviro-nazis were demanding things that would break down into simpler compounds once placed in landfills. Two decades later, they learned that these simpler compounds they had demanded were now leaching into our water supply. Idiots. So yes, locking up carbon in landfills would be the logical approach. But logic simply isn't their strong suit.