That is true of the only some of these pictures. I have seen similar photos of a B2 which is definatly sub-sonic. In that case the airflow over the wing experiances a drop in pressure which causes the same vapor to form.
Of course all airplanes experiance this differential pressure across the wing section. Bernouli's therum dictates that this is how a wing generates lift. I expect the the cloud formation (either sub or supper sonic) has more to do with the degree of saturation of the air. The closer the ambient temperature is to the dew point, the more likely that a pressure difference will form vapor.
Regards,
GtG
PS Prandtl-Glauert singularities generly form at the front tip of the fusilage and along the leading edge of the wing.