Clarification of the problem:
Your misinterpretation of what a bounty is, and should be used for.
A bounty is designed to draw attention to a question, in the hopes of getting an answer so that if an answer solves it a user may get awarded the bounty. (Or, for those with excess reputation and can therefore afford it, to provide an incentive for others to answer a question, or to reward a good answer).
It is not designed for 'adoption' of a question, and you are not entitled the same permissions as you would have by posting the question yourself as a result. This ultimately means that to interact with a post you've put a bounty on, you would not open a bounty that would revoke the permissions you need for interaction with that post.
To quote Jeff Atwood's post on Meta.SE, on the question about splitting bounties from affecting permissions:
... bounties are for people who either
- feel like they have an excess of reputation and can "afford" it
- have an urgent need to get an answer and the normal process needs goosing
This does not fit your 'using bounties to adopt a question' view, and by design it cannot support that view.
The unbreakable bond between Reputation and Permissions.
As stated in the Help Center, and elsewhere, including on Meta.SE, posting a bounty will affect your reputation level. It will also reduce your permissions based on the reputation level.
As was indicated by Jeff Atwood on Meta.SE, in regards to the Feature Request (on Meta.SE) to Exclude Bounties from Affecting Privileges, there's no really good way that can be thought of to separate bounties from the reputation system in a way that would allow for permissions to be retained when a bounty is posted; as such, it's unlikely to ever be implemented.
With this in mind, the feature request here is asking to permit commenting on awarded bounties, outside of the confines of the existing permissions system. This ultimately ties back to having to exclude bounty awards from reducing your permissions levels, because to comment on any post that is not your own you need to have 50 reputation. With no mechanism to separate the permissions, the feature can't be implemented without changing how permissions are affected by bounties. And Jeff has said this is unlikely to get implemented, which ultimately points back to the "Declined" status, and the "This is a problem with the existing system, and not one that is likely to be fixed/changed" nature of this answer.
NOTE: This has already been asked and answered and addressed on Meta Stack Exchange. Twice.
What exists below is the opinion of the moderator who answered the question here. This opinion is superseded by the post up at Meta Stack Exchange.
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.