It's called stealing, because that's the society we live in. We don't like in a Freeganism society so it is illegal (i.e. against the law of this country that we live in).

What if you "borrow" an item with full intention of returning it. The other day I needed a ladder to do a job.  I didn't want to buy a new one - the carbon imprint of manufacturing an aluminium ladder must be massive - but the chap at the diy shop was none too pleased when I tried to stroll out the doors cheerily saying "I'll have it back by noon tommorrow"

What they are known for is getting food from rubbish bins. That is illegal in the UK, it is considered stealing. Stuff shops throw away is still their property until the refuse collectors pick it up, and then it becomes the property of the refuse companies.