But who decides what comprises "wasting [my] bandwidth"? This is subjective.

Is it me (the site owner in this example)? If so: Since it's my subjective decision to make, couldn't I equally legitimately decide that traffic I serve to non-human entities is "wasting my bandwidth"?

To be clear, I'm not trying to make the case that there should be some law in place that prevents scraping or machine access across the board -- only that it would make sense for website operators to be able, optionally, to include that kind of usage restriction in an ordinary contract and legally enforce it by the usual means (lawsuits), in addition to any kind of technical restrictions they are able to put in place.