Landfills amortize the carbon emissions. Keeping the plastic's carbon temporarily sequestered in a landfill puts it somewhere other than the atmosphere. Eventually we'll have to do something with all that carbon, but we're better off burying it until we get a better handle on emissions in general.
You would be right, IF we were to not take out any other fossil fuel to burn. But to bury the plastic now it just means we will dig for an equivalent quantity of coal, oil or gas to burn. We will burn some stuff regardless, we might as well burn what we already have, instead of burying it and digging for new stuff.
