I think this is a weak framing. Lots of things are moral or immoral under specific circumstances. We should protect people from being murdered. I think murder is usually wrong. But we also likely agree that there are circumstances in which killing someone can be justified. If we can find context for taking a life, I'm quite sure we can find context for a DoS.
So their desire to not be used to commit a cyberattack doesn’t factor in? As long as they aren’t legally liable, it doesn’t matter?
Also a checkbox that says something like “I would like to help commit a crime using my internet traffic” would keep people from having their traffic used without consent.