It is perfectly legal to take photographs of anything that you can see when you are in a public place, or, a publicly accessible location.

This might be upsetting for some, so for many, discretion is often used. It is, however, up to the photographer. 

The only time the photographer has to be aware, is in situations where privacy could reasonably be expected, such as toilets, changing rooms, bathrooms, or, through windows of a residence.

See Not An Auditor for more information.