If someone broke into the KFC vault and wrote down the spice recipe used for the chicken, we'd still call that a "stolen recipe". If part of the value of the source code is its secrecy, then its value decreases when it's made public.

Look at an example of one way the word "steal" is used in speech. If I say "Good artists copy; great artists steal", and I saying that great artists break into a building and illegally remove a physical artifact, or am I saying that they copy something for their own benefit? If one can "steal" an idea, then isn't that a "stolen idea"? And if that stolen idea is directly used to create some salable product, then isn't that a "stolen product", in that sense?

edit: The comment I responded to made the claim that source code couldn't be stolen, only copied (similar to the standard argument "it's copyright infringement, not theft", often applied to copied media). There was more, but I don't remember the wording, and I don't want to misrepresent the position.