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.
I took the following picture in Paso Robles, CA on 4/26/2013.
You’ll be happy to know that, since then, I dumped my psychoactive medication in the drain to the sound of Michael W. Smith’s Healing Rain.