I think BI proponents argue that specifying what counts as a contribution leads to economic inefficiencies, in the broadest sense. Does taking care of your own children count as a contribution? Or sitting in the pub and making other visitors happy by your presence? What about writing software and putting it on Github? And for the latter, should you get a minimum number of stars to make it count? How long should the list of "useful activities" be, that the Department of Civil Contributions would maintain? And how much money would we spend on civil servants who determine whether your edge case qualifies?
The result would be that people end up doing artificially much of what counts as "contributing", and too little of some things that aren't specified as "contributing", but that society actually still benefits from.
Would some people just sit on the couch all day, and really contribute nothing, by any standard? Yes, probably. So the premise here is that the number of true freeloaders is small enough, that it would be inefficient to worry about them. I personally don't know any true freeloaders, which leads me to think that that's probably a fair assumption.