How would one “steal a kiss” if it was a finite resource.

Stealing a kiss is another poetic use of the term.

Also while stealing is not always “punishable by law”, it can still be classified as wrongdoing.

Depends on the context.

As an artist I find all forms of plagiarism to equate to stealing

As an artist, I don’t. Plagiarism is about failure to cite sources and copying content while claiming it’s yours. That’s not even usually a legal issue. It’s an academic issue. And it’s not necessarily immoral either. You can be accused of plagiarizing yourself by not citing that you had previously written some of the content in a previous work. That’s not even close to stealing, technically, legally, or morally.

Plagiarism is also very different from copyright violation.

and while they won’t land you behind bars, they may get you a status of persona non grata with other creatives.

And it might get you fame and fortune and acclaim from others. Depends on the context.