Because the geometry of the model, if it is one that came with the Poser Program, or is owned by Daz3d, belongs to them. Now if it is one created by an individual, and is not a modified version of one of those owned by Curiouslabs, Daz, etc... then it is up to the individual owner of the model. Make no mistake though, to export out to other programs the models that came with Poser, or those that you buy from Daz, modified or not, is in fact stealing, unless the persons using the export also owns the originally licensed versions.