I think a distinction needs to be drawn between "inappropriate" and "shocking". Shock is about the magnitude of anticipated response, whereas impropriety is about things being out of place. They can go hand in hand (the most inappropriate things usually have a shocking aspect), but they don't quite work in the same way.

If you want to maximise shock, you transgress the biggest taboos you have available, but if you transgress too many at once it stops being shocking and starts being farcical. If you want to maximise impropriety, you need to provide something tailor-made to transgress the specific taboos of the situation you're in. By way of example, if a gynecologist wolf-whistles while giving a patient a pelvic exam, that's inappropriate. If he stabs her to death, that's shocking. If he chops her up into bits and feeds her to orphans working in his sweatshop that manufactures clubs for killing baby seals, that's farcical.

The contradictory nature of "fuck the police" in swastikas is somewhat farcical, making it irreverant about serious issues in a way that isn't as bluntly shocking. This is probably the "cancelling out" you mention. It could certainly be more shocking, but it's supposed to be out of place, not to generate a response of high magnitude.