Not having the required 50 rep for the privilege of commenting still allows commenting our own posts (and the answers of our own questions).
This exemption obviously should be extended for the rare cases, as the question is not ours, but we have an (active or inactive) bounty on it.
The rationale that rep<50 users can't comment everywhere, is that first we have to produce something, and only after should start chit-chat. The few ones reached 75, and then started a bounty on others question, have produced something already.
Another reason, why rep<50 users can comment the answers of their own questions, is that so they can request clarifications from the answerers. These clarifications can play a significant role in the decision, which answer do they finally accept.
The same rationale similarly exist, if the question is not ours, but we want to request clarifications, which can play a significant role in the decision, to who do we want to give the bounty.
A possible workaround for the problem:
As you have at least 25 rep after the bounty, you still have access to chatrooms. Thus:
- Start a chat in a new room, or in a relevant room about the bounty
- and put its link into the bounty notice, or in a comment below the question (before starting the bounty).