USPS is complicit.

It's not illegal to use mass mailing like this to random people. What about all the coupons or weird "for current resident" junk mail that exists? I get a chunk of mass mailers every week. During election season all sorts of organizations, not strictly the candidates, send out these kinds of information postcards. If you pay for the postage and go about it a legal way, the USPS will mail the kinda postcards all over for you, pretty much regardless of topic as long as it isn't explicit I'd imagine.