We study the disks of B[e] stars assuming that the disks stem from the angular momentum loss from the central object. The angular momentum loss may be induced either by evolution of the stellar interior of critically rotating star or by merger event in a binary. In contrast to the usual stellar wind mass loss set by driving from the stellar luminosity, such decretion-disk mass loss is determined by the angular momentum loss needed to keep the central object in equilibrium. The angular momentum loss is given either by the interior evolution and decline in the star's moment of inertia, or by excess angular momentum present in a merging binary. Because the specific angular momentum in a Keplerian disk increases with the square root of the radius, the decretion mass loss associated with a required level of angular momentum loss depends crucially on the outer radius for viscous coupling of the disk. The magnetorotational instability can be the source of anomalous viscosity in decretion disks. The instability operates close to the star and disappears in the region where the disk orbital velocity is roughly equal to the sound speed. We study the differences between Be and B[e] star disks and discuss the reasons why stars of the stellar type B have disks, while other stars do not.