An interesting article I found on the web. I think the points are as follows:

Don't take grease without getting permission;

With rising prices, there is now value in used cooking grease. Restaurants may stop giving it away.

http://www.nytimes.com/2008/05/30/us/30grease.html?_r=1&pagewanted=print

It's nothing new, it's just getting media attention now. Five years ago when I was living in Edmond, OK there was a big story about how someone stole 300 gallons of WVO from a big restaurant there. It wasn't me but a lot of my friends gave me trouble about it. Back then, very few people knew why someone would even want that much grease. Now, renderers are taking this sort of thing very seriously.

Why steal if you can ask for it?

Maybe you get turned down the first couple of times, but if you continue to visit to restaurant eventually they will say yes, just take it.