There was a discussion about this in the airliners forum. Precisely because the fuel cannot be reused for aircraft use until tested extensively, and airports don't have spare holding tanks available (so they've got to give up a bowser, which needs to be cleaned and tested afterwards), and defueling takes ages, it is cheaper to just burn off the excess fuel than to remove it.
As for removing fuel from a GA aircraft - you can siphon it off or use the fuel drains. Both will be somewhat messy and take some time. Fortunately our club doesn't have a "refuel after flight" policy and it rarely happens that a plane has too much fuel on board. In fact, on the first page of the log book of our R2160 (the only aero plane in the fleet) we've now written in bold letters never to return the aircraft with more than 1/2 fuel or otherwise it'll be out of the aeros W&B envelope. You can't miss that warning, as it is right next to the plastic sleeve that holds the fuel credit card.