Sorry mate you are conflating two distinct issues as if one is the cause of the other. FON is a long established principle that predates UNCLOS. The underlying problem in the SCS is because there are competing claims and unresolved questions of jurisdiction. China's nebulous claims and unilateral application of rules is adding confusion to the picture. The CNN overflight story was a window to the world as an example of China's attempt at restricting FON. It is no accident that the story was captured by CNN as one would easily conclude previous attempts of restriction was happening and CNN's presence was to showcase to the world what was in fact happening with FON.
It is a problem because China made it a problem and not because of FON.
The P-8 overflight using a military aircraft was warned via the use of a small military exclusion zone. Do you believe that warning is proportional to potential denial of FON for civilian vessels and aircraft over the entire SCS?
The CNN overflight was actually perfect in describing the Chinese approach where the civilian airline requesting clarification was reassured that the messages were not directed to them, showing the dichotomous Chinese approach between civilian versus military presence near its territories and claimed territories.
That is about a differing issue in interpreting a particular clause within UNCLOS relating to military assets and surveying (see the Impeccable incident), unrelated to
I agree that China's territorial claims as we understand it are not very clear, however at the same time have there been any instances where China has sought to restrict FON of civilian vessels or aircraft in a manner unrelated to territorial disputes?
So no, the problem is not about China seeking to restrict "FON", but about military power and the desire for different countries seeking the ability to project it or restrict it.
At the very least, throw me a bone and admit that issues where "FON" has been challenged were all relating to the navigation of military assets (USNS Impeccable, the Cowpens cruiser, the P-8 incident), not civilian, in other words, I will concede that China may potentially seek limited restrictions on the navigation of certain military vessels near its claimed territories in the SCS (but this is not without precedent that some other nations believe in -- re: the article the Shen posted regarding differing international interpretation of EEZ surveying and military presence).
However I will resolutely argue against the idea that China would seek (or has sought) unconditional restrictions on civilian freedom navigation in SCS (and challenge anyone to find evidence of such desires), and also argue against the blatant all encompassing phrasing of "challenging freedom of navigation"... when it should more accurately be described as "potentially challenging the ability for certain military assets to navigate within the scope of claimed territories".