I think you are mixing two things. The outcry seems to be from one business copying another business. It would be like one songwriter copying the work of another songwriter. You are then comparing this to a regular person copyrighting the work of a songwriter/singer for their own consumption.
Although they are similar they are not the same thing. Just wanted to point that out. I won't go into an argument about which is wrong though since that is not the intention of my comment.
I just want people to stop using "theft" when "copyright violation" and "unauthorized usage of resources" (which is closer to theft than copyright infringement, BTW) are the right expressions to describe something. For the pure copying of substantial parts of code from one site to the other, it's not that much different from one musician sampling another one's work. Our HTML, JavaScript and CSS are all out there exposed and readable. I fully expect to find parts of it copied all over the web. It's a fact of life. I don't always like it, but I know better than wasting my time with it.
I don't think Curebit has any right to use 37 Signals' work, far from it, but I also think this outcry is a bit of an overreaction to mindboggingly idiotic deeds made even worse by a poorly conducted response.
Once, a friend of mine caught a competitor using their CSSs and images (directly off my friend's servers) on their own site. One morning, the competitor woke up to find their site completely disfigured in randomly clever ways for every IP that wasn't their office's. It was very enjoyable to observe.
Although they are similar they are not the same thing. Just wanted to point that out. I won't go into an argument about which is wrong though since that is not the intention of my comment.