Well, @2fire could explain it truthfully, of course; just a general remark 2 cents worth:I was not aware of this. I have two Robots running every day also Sunday, they make as much noise as a normal vacuum cleaner, so far no one has complained. I live in a DE Cantone.
I do not think that any such restriction is imposed generally. (It would not make any sense e.g. on the remote farm.) But there is a lot of similar restrictions concerning particular regions, towns, town districts, apartement houses... So it's pretty possible that in the place where you live this is, by coincidence, completely allowed