Maybe the best compromise is just to show a warning, and allowing the weak password afterwards.
That could be a good option, as the warning already present for the default password (if I remember correctly). At the other hand... how complicated is to set a "strong" password? Just to have enough characters, including numbers, symbols. And if somebody use the same password on different machines... that's his own and private decision.
And something else about the default/weak passwords... These machines usually are connected to internet. In the case of the weak password, what's the chance to get it transformed in to a "zombi" machines? To be used it by others for something "bad"?