I cannot speak for this particular airport, but if I had to guess, I'd say that the displaced thresholds provide the required approach angle clearance. I have seen cases at some US joint military and civil airports where there were arrestor cables in the displaced threshold areas, but it does not appear so in this case. If the load bearing of a certain portion of the runway is reduced, it's going to make that pretty clear. I have found that generally (though not always) its permitted, and actually desired, that you begin your takeoff in the displaced threshold area. It provides a greater accelerate stop distance, and gets you higher over the neighbours on departure. Read the rules for the airport...