This question already has an answer here:
According to the answers here at meta, you can lose privileges when you e.g. spend your reputation on bounties. What is the rationale behind that?
By my opinion, it would be more natural if the privileges are persistent once gained. The user is given privileges once he was "tested" by the community. He has to spend some time at the site to gain reputation and see how it works. We consider him trustworthy then.
Once the person is trustworthy, what makes him untrustworthy when he spends his reputation on bounties? Does he become a beginner then? Will he suddenly forget everything he learned at SO?
P.S.: I would understand it for negative points from downvotes (or penalties, if there are any). But when placing bounties.... or when your answers are migrated (I don't know if you lose points in this situation) I don't really see any reason to lose privileges.