It is a common misbelief that if you want to do X, X is all you need to know. Every profession comes with a certain range of basics that are essential even if they are not necessary in everyday life (for that profession).
For example: I studied math, but even if I studied informatics, I would've learned about Taylor series and similar things. Things that literally never so much as come up in my everday job.
But it's not always just about the knowledge of the subject itself, but also about knowledge in other subjects and, often more importantly, the skills these other things teach you.
Ownership in this case doesn't matter. If that was a "way out" of cheating, I could've bought my final thesis from soneone. But I couldn't. And why not? Because educational institutions demand that you actually created the work you hand in yourself, not just that it is legally yours.
Helping someone to cheat may not be illegal and heck, I don't even care if it's morally wrong. The one thing you can not argue your way out of is that it is not their own work – and that is exactly what schools demand.