Basically, the artificial flavors are really cheap, so people try to avoid the natural stuff when customers don't care.
But on that note, there are some good ingredients, since some flavors are too hard to create unless you use the real stuff (like black tea -- you just overbrew it to make a concentrate).
What I think is funny is when an ingredient can both be found in nature (e.g. in the shell of a beetle) or made in a lab, but the generally more expensive and lower quality natural version is chosen so the product can be "all natural."
Personally, if beaver butt is tasty, that's fine with me, but I respect the desires of other to avoid animal products, and think it's reasonable that such information be clearly disclosed.
Disclaimer: I didn't RTFA, and it's entirely likely this argument was made there.
A couple of PDFs on the subject:
Perfectly natural, but certainly not what you would expect these flavours to come from.
In that context I can see really nothing strange about using fungi for flavouring.