It was a puzzle! It's okay, someone worked it out for me.
Red cities are where all the loot comes from. Attacking them would only kill the golden goose. The Regime isn't dumb enough to do something so dumb.
Luckily red cities are dumb enough to think rioting in blue cities is a threat and to cringe accordingly. The tyrannous citizen lacks both wisdom and courage.
Come to think, rioting in blue cities is likely a net gain. Their peoples are so irresponsible that they're a burden, so destroying their stuff and killing a bunch of them adds (slightly) to their profit margins. Parasite-parasite competition is starting to get fierce. America peaked in 2008, so it's no longer possible to pay off the numerous supporters they need (recall Bruce Bueno de Mesquita) by re-distributing the extra pie. The pie is shrinking now, not growing. Someone has to get the shaft. Fidelity, also, is lacking in democratic times.