Actually, they do forcefully have dunmeri spirits protect things. And more than likely, it is going to be a member in the family that did nothing with his/her life, got into trouble a lot, and/or were more of a burden or waste of space than anything. And the guardians are definitely not protecting graves and tombs on their freewill, oh heavens no. They are forced to guard tombs, crypts, and vaults, and they have absolutely no say.
Plus, necromancy doesn't really deal with spirits. They're a take a husk of a body and make it do things. Basically, they're puppeteers, and the skeletons and zombies are their puppets. Without someone to pull their strings, bones and cadavers just lie around..rotting.
Taken from "Ancestors and the Dunmer":
However, as a sign of great honor and sacrifice, an ancestor may grant that part of his remains be retained to serve as part of a ghost fence protecting the clan's shrine and family precincts. Such an arrangement is often part of the family member's will, that a knucklebone shall be saved out of his remains and incorporated with solemn magic and ceremony into a clan ghost fence. In more exceptional cases, an entire skeleton or even a preserved corpse may be bound into a ghost fence.
Or maybe Ive just got guardian spirits and ghostfences mixed up?
Spirits that are forced to remain in our world against their will may become mad spirits, or ghosts. Some spirits are bound to this world because of some terrible circumstances of their death, or because of some powerful emotional bond to a person, place, or thing. These are called hauntings....
Some spirits are bound against their wills to protect family shrines. This unpleasant fate is reserved for those who have not served the family faithfully in life. Dutiful and honorable ancestral spirits often aid in the capture and binding of wayward spirits.
This states what youve said is true, atleast to some extent, that some are forced to guard duty, but the fact that it says "SOME spirits are bound to..." indicates that not all of them are.
Maybe I just got it wrong :shrug: But afaik theres both those who do it by free will and those who are forced to do it. And even if you are forced, I consider it to be more justifiable to be forced to protect your family instead of some complete stranger who needs someone to do his dirty work. But I can agree on the puppeteering though, if the body is dead without any kind of spirit, then theres nothing wrong with it unless its stolen.