Google, commenting on the change, says its advertising policies are "developed and evaluated based on multiple factors, including legal and cultural considerations plus user and customer experience".
Plagiarism/copyright infringement are legally actionable, but it looks like legality isn't the only reason.
Submitting a bought work as someone's own is fraudulent, and while it may not be illegal, it doesn't quite measure up to being culturally acceptable. Ultimately, the end user is the universities, who use the papers for grading purposes and graduation requirements. As the end targeted recipients, it doesn't exactly provide them with a satisfactory user experience.