Earlier quoted context omitted.
Having a front door physically allows anyone on the street to come to knock on it. Having a "no soliciting" sign is an instruction clarifying that not everybody is welcome. Having a web site should operate in a similar fashion. The robots.txt is the equivalent of such a sign.
No soliciting signs are polite requests that no one has to follow, and door to door salesman regularly walk right past them. No one is calling for the criminalization of door-to-door sales and no one is worried about how much door-to-door sales increases water consumption.
Ok, I am, right now.
It seems like there are two sides here that are talking past one another: "people will do X and you accept it if you do not actively prevent it, if you can" and "X is bad behavior that should be stopped and shouldn't be the burden of individuals to stop". As someone who leans to the latter, the former just sounds like restating the problem being complained about.