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.
And, despite what ideas you may get from the media, mere trespass without imminent threat to life is not a justification for deadly force.
There are some states where the considerations for self defense do not include a duty to retreat if possible, either in general (“stand your ground" law) or specifically in the home (“castle doctrine"), but all the other requirements (imminent threat of certain kinds of serious harm, proportional force) for self-defense remain part of the law in those states, and trespassing by/while disregarding a ”no soliciting” would not, by itself, satisfy those requirements.
>No one is calling for the criminalization of door-to-door sales
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.
