Some fallacies used by defenders of plagiarism*:
ad hominem (against those accusing the plagiarist)
tu quoque (everyone does it anyway)
argument from the motivations of those pointing out the plagiarism (if what they say is true, it doesn't matter, because their motivations are tainted)
canards and red herrings, various kinds of question-begging
argument from authority (the plagiarist is a good guy / woman in other ways, a prestigious scholar, etc...)
confusion of copyright in the legal sense and the ethical code of scholarship
argument from the percentage of the total output affected (only 2,000 words in a 50,000 word book)
argument from sloppiness, the non-intentional nature of the offense
argument that the plagiarist is a smart person and thus didn't need to plagiarize; therefore s/he didn't
argument that a student would have had to do what the plagiarist did a few more times to be brought up on charges
argument that unattributed paraphrase is ok if you "change enough words"
argument that direct quotes without quotation marks are ok if the original author is cited a few paragraphs (or pages) away
argument that none of the people plagiarized has complained yet (!)
argument that we live in a digital age
argument that 8-year olds will grow up to be plagiarists because of wikipedia
*Adopted from my summary of defenses of Frank Fischer on a CHE comment board.
Update: One more: you are not in this particular field or subfield, you don't know enough about its mores, its secondary literature, to make an argument about plagiarism.