Of course its stealing!
I actually did this once, but it was accidental and I totally apologized and put the money back. I was in a Starbucks and they were using a clear plastic square with totally open top, sides only about 3 inches high, as a tip jar. It only had a small handful of change in it (maybe they had just emptied it?) My total came to something like $2.03 and I had no change, so I dipped into the jar - I honestly thought it was a "take a penny, leave a penny" dish. The look on the employees face made me question, and as soon as he confirmed no it was tips, I just pulled out another dollar. I even left some of my change as a tip out of guilt.
I do think the jar needs to be well labelled (in the letter it says it is) and it should have a lid with a slot cut into it, like a piggy bank, to deter theft. A "take a penny/leave a penny" dish is not a bad idea either if there is room on the counter for both.