No idea, really, but generating electricity by burning plastic might release less carbon than from burning coal because the plastic contains hydrogen as well as carbon. It depends, no doubt, on the type of plastic and the efficiency with which it can be burnt while avoiding the release of nasty chemicals.
It's not at all obvious to me what's best: landfill, burn, or recycle. But government policy should be based on a proper analysis of those options. I have the impression that government is mostly just compromising between what industry lobbyists are asking for, in order to maximise their profits, and what the ignorant public are asking for, in response to fashion and irrationality. Let's hope that somewhere some sensible civil servants are quietly doing the right sums.
