The reason witnesses have the finger on that parameter is because it may need to be dynamic (depending upon market conditions).
Technically committee members can also publish feeds, but because committee members are not PAID they do not have income (nor skills) necessary to setup a VPS system to publish feeds.
Witnesses are also "witnessing" the price, and "testifying" as to the feed.
We wanted to have "voting" on this parameter and using the median of the feeds is a way of voting. No single witness can manipulate this parameter.
Also we wanted it be easy to change quickly, and committee actions normally take a 2 week review period.