Non force sensitive purebloods can become slaves/normal citizens. Force sensitive purebloods are held in high regard as I said force sensitive purebloods are treated like pedigree dogs.
Girdeux the reason why it does not make sense because various lines said by the characters within the SI story does not make sense and does not mesh well with the pureblood race. Harkon calls ffon pure when you are clearly a pureblood as well. Plus when zash mentions her vision and says that the SI would appear from low/humble birth and your a pureblood so that does not make much sense.
Its things like that yes purebloods can be slaves but you being a pureblood in the SI story does not make sense in context of the SI story at various points. It was not written with them in mind the Si story was orginally written for aliens and humans. This is fact.
Indeed, although purebloods, even non-force sensitives, cannot be slaves. Or at least their is no evidence to suggest that they are not as well treated as force sensitives. They often get important positions and are still held in high esteem, such as the pureblood in "Revan' got a high place partly because he was pureblood. Executor Krannus was part of the Emperor's elite servants etc.