The modifiers were put in the Constraints category because the Blender Modifiers category does not appear for non-visible objects like empties and cameras. Since LinkInPoint modifiers, for example, are generally attached to non-visible objects, the only option was to put the Plasma Modifiers in Constraints. Putting them in the Object category was considered but rejected because the Object category already has enough panels in it.
Edit: I'd also like the mention that the physics bounds options were taken directly from the names of the Plasma flags. In the future "explicit" may be merged with "proxy" for the sake of convenience. If this change occurs it will probably be connected with a major addition to the physics code, most likely when the convex-hull type is added.