Why did 'verboten' come into use when we already have 'forbidden'?
In addition to agreeing with the excessively-prone-to-follow-orders-without-question connotation FumbleFingers mentions in his comment, I believe that the use of "Das ist verboten", with an over-the-top stress pattern, as a mild reproof hedges it: "That is forbidden" or, more commonly, "You mustn't do that" sounds far more censorious. The pomposity is now partly self-acknowledged, so the person being told off doesn't feel so cut off as the only non-normal person.
Perhaps this only works when the recipient has watched " 'Allo 'Allo " or the like.