Because when money from the state budget is spent on research you expect all of it to go into, you know, research. That means labor and material costs for people actually doing the work. Scientists, lab assistants, reasonable amount of administration.
Publishers are effectively stealing money from taxpayers. Research benefits the whole world but Elsevier's profits only benefit its shareholders. Every million sent to Wall Street funds could be used to start a project that might end up saving lives.
Why that particular combination? You could just as easily compare private spending and public benefits, and it would make about as much sense.
Maybe better to include both public and private costs and benefits?