I believe that loss of privileges is implemented to prevent abuse of the system, and/or is a limitation of how reputation-based privileges get handled.
That said, it is clearly stated in what bounties are what happens if you give a bounty and drop below a reputation privilege level. This has been the way the system has worked ever since I joined up here, and I don't disagree with it. I also believe that this "loss of reputation and privilege" helps to discourage abuse of bounties just to boost someone's reputation in a malicious way.
With regards to the loss of privileges, I am going to quote the Help Center on the Main Site:
What is a bounty? How can I start one?
...
- All bounties are paid for up front and non-refundable under any circumstances.
- If your new reputation brings you below the requirement for any privileges, you will lose access to those privileges.
- Users may only have three active bounties at any given time. Questions may only have one active question bounty at any given time.
- To avoid overly promotional bounties, if you are offering a bounty on a question that you have already posted an answer to, your minimum spend is 100 reputation (not 50).
- Additionally, if you offer multiple bounties on the same question, the minimum spend doubles with each subsequent bounty (50 reputation on the first bounty, 100 reputation on the second, 200 on the third, and so on). You may not cancel a bounty once it has been started.
(Bold-italics are added by me, for emphasis, and are what I'm focusing on)
I can see why you would still want to comment on user posts of which you've made a bounty for, and retain all your privileges you had, but because I don't know the underlying limitations to the system, I can only say that I agree with the status quo.