I think having a "photography allowed" sign sets a dangerous precedent. Right now, you are allowed to take photographs anyplace that is not specifically disallowed by law. If these signs (or something like them) become ubiquitous it becomes only a small step to "you can only take pictures where you are specifically allowed to".
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.