[alephnil]

Those robots at least stop at the boundary of the field, unlike fertilizer and pesticides, that run off to the surrounding environment.

Such robots are mostly on a prototype stage, and not applied widely in agriculture today, so they are a possible problem of tomorrow rather than a present problem. Fertilizer run off is very much a present problem.

Also, the robots will typically replace a combination of herbicides and manual labor. It is not like farmers let the weed grow freely today. As such they may be an improvement over current practices.