It's meant to only be placed next to signs forbidding photography so you shouldn't see this sign by itself. I think its a good way to protest those signs and doesn't set a dangerous precedent if only used as a reaction to "photography forbidden" signs.
Much better, I think, to familiarize yourself with actual law (here's a good writeup for people in the US[1]) and to push back against mis-informed people trying to tell you what you can and cannot do.
[1] http://www.krages.com/phoright.htm