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.
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.
Of course, when it comes to official signs. But this seems to be intended to be stuck up next to 'no photography' signs to counteract them. I kind of want one of those shirts.
"As a matter of fact, you can shoot pics pretty much anywhere: in subways, airports, transportation centers, and malls."
Advice like that will create photographers that have as poor an understanding of the laws and are as misguided as most of the security guards that hassle them.
Specific regional by-laws aside, photography of anything in public places or private places (without a reasonable expectation of privacy) from a public place is usually fine, but there are typically exceptions. If, for instance, you're in a privately owned public space (eg. a mall), it's fine to take photographs, but if a security guard or some other custodian of the property asks you to stop, and you don't, you are then trespassing. Likewise, in the city I live in (Toronto) it's prohibited to take commercial photographs on the TTC (subways) or in public parks without a special permit. This is to prevent such things as, say, an overzealous photographer taking over a park to get some wedding party photos.
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