Well, because churches are often against social progress. So when they liken social progress to Satan, in their minds they are demonising social progress, while for you (and me) it sounds like they’re saying Satan is socially progressive (which is a good thing).

Because churches that fixate on Satan and going to hell and whatnot are also the quickest to make strong moral judgements about what is “evil” and tend to include a lot in that category, including stuff that is normal and universally accepted. Most churches don’t talk about Satan at all, so their billboards don’t have this ironic side effect.