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.
The implicit message here is that anywhere that this sign does not exist, photography is not allowed. That's a bad precedent to set.
Imagine walking around and seeing "Free Speech Permitted in this Area" signs.
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.
It's obviously just thuggery to disallow photography in a public place.
The making of the Istanbul traffic lights