Good question! Why is there a fuel surcharge at all?
In the days when airline tickets were often priced anything up to a year ahead, volatility in the price of fuel made it very difficult to accurately predict the dispatch cost of a flight. Fuel surcharges were a method that in some measure helped to smooth out this distortion. However it is extremely rare to recall there ever being a fuel rebate applied? These days when yield mangement enables an airline to price a ticket almost minute by minute how can it a apply a defined surcharge to a cost it largely already knows. Particularly so when much of that fuel has already been purchased on the futures market.
The answer is simple. The regulator who would come down like a ton of bricks on most retailers engaged in this form of "fluid retailing," seems reluctant to seek legislation to prevent it. Thus companies can continue to sell a £99 product for a headline price of 99 pence, and attempt to claw (no pun intended) the real cost back through the fuel surcharges, taxes & charges, booking fees, etc.etc.
Fido of course is oblivious to all this because he doesn't have to pay the bill.