If plagiarism is theft, then it does matter whether the original author cares you've plagiarized her. As Thomas comments on this post, however, the damage of plagiarism is not to the original author (primarily) but to the reader and the institution of scholarship itself. We can see this easily in our classes. We punish plagiarism even though it does no damage to the original authors, who usually don't even know. If someone plagiarizes wikipedia, we don't care how wikipedia feels about it. If the author is deceased, or is cool with being plagiarized, we still call it plagiarism. If I write an article and sell it to you, and you publish it under your name, no theft has taken place, but it is still a breach of academic integrity, just the same. That is because the harm is to the reader.
Of course, plagiarism may involve intellectual property issues, and often does, but it is not primarily a matter of IP. I can plagiarize material in the public domain, that nobody owns. I can plagiarize material or no ostensible economic value, like the typical academic article, and I cannot make it right by paying the original author the price of a beer, even if the author agrees to this.
So plagiarism may be a form of theft in some cases, but that's not the main reason why it is wrong.
Similarly, the defense that the plagiarized paper is cited by the plagiarizer is not a legitimate defense. You often hear people say, "well, look, the paper is cited, so clearly no plagiarism has taken place, or wasn't intentional." If you don't attribute specific words and ideas to their rightful authors, it is plagiarism, even if you cite the authors a dozen times for other words or ideas. In fact, a large number of citations to another paper, combined with an even larger, but concealed debt, is especially pernicious.
You often hear people reluctant to care about plagiarism. To me, that is like not caring about scholarship itself. If it doesn't matter to you who said what when, then what are you doing in the business?