The shark fin was required as an FAA official felt the machine had too much directional stability. How he managed to get that through as there is very little in the certification requirements about lateral directional stability is beyond me.
The issue was that with too much directional stability, the machine might be prone to negative spiral mode (i.e. it would roll into a turn or, put another way, require out of turn cyclic to maintain an angle of bank).
Rumor has it that when he retired, the shark fins could be removed... Hopefully someone will correct that if it's wrong.