Banksy said it better:

People are taking the piss out of you everyday. They butt into your life, take a cheap shot at you and then disappear. They leer at you from tall buildings and make you feel small. They make flippant comments from buses that imply you're not sexy enough and that all the fun is happening somewhere else. They are on TV making your girlfriend feel inadequate. They have access to the most sophisticated technology the world has ever seen and they bully you with it. They are The Advertisers and they are laughing at you. You, however, are forbidden to touch them. Trademarks, intellectual property rights and copyright law mean advertisers can say what they like wherever they like with total impunity. Fuck that. Any advert in a public space that gives you no choice whether you see it or not is yours. It's yours to take, re-arrange and re-use. You can do whatever you like with it. Asking for permission is like asking to keep a rock someone just threw at your head. You owe the companies nothing. Less than nothing, you especially don't owe them any courtesy. They owe you. They have re-arranged the world to put themselves in front of you. They never asked for your permission, don't even start asking for theirs.