Ok, then change the situation to an outdoor drive-in movie theater. I own the property next door and I can watch drive in movies for free without paying the owners, and without exhausting any resources.
That's not theft in my opinion. You are not required to avert your eyes. ;)
Or, just take the issue of mp3 sharing over the Internet. Both of these involve non-exhaustibility and non-excludibility. In other words, both involve no harm other than possible lost opportunity costs.
That's a trickier issue (to me, anyway. The whole thing seems pretty black and white to you, I gather). I haven't really paid a whole lot of attention to the whole MP-3 debate. The technology is such that I think we're not just talking about "possible lost opportunity costs" but almost certain lost opportunity costs. I.e., potentially thousands of people downloading music that they WOULD otherwise have purchased. Some might not have. It's impossible to quantify, but I do think there is harm here to the record companies (loathsome though they are) and the artists.