They could and have done this for a while. Used to see it in the blackhat marketing space, where content farms wishing to rank higher than other farms spammed their competition. Was glad to see the content farms go circa 2011 (Panda update).

Hate giving people ideas, but a majority of the fiverr SEO services are geared towards spam/link bombing competitors.

Maybe that's not the intended use of the "SEO" services, but I fail to see what else they would be good at.

Exactly, and now it puts the onus on the website owners to make use of the disavowal tool, which can only be done with extra monitoring on their part.

Does the disavowal tool negatively affect the sites being disavowed?

It 'shouldn't otherwise it too could be used as a tool for negative campaigns. I would assume though that it will be used for looking at trends, footprints and other methods of better refining the detection algorithms.

My thoughts exactly.