Here's the reasoning at our operator - We lease the engines on a 'Power by the hour' agreement from the manufacturer. Prior to 1000' AGL the manufacturer gives credit for de-rated or assumed temperature takeoff power reduction. That equals less cost to the airline.
Above 1000' AGL these 'credits' end. At this point it saves fuel to climb to the optimum cruise altitude as fast as possible using maximum climb thrust. Again, equalling less cost to the airline.
This is true for a CFM-56-7 power plant under a power by the hour agreement on our 737NG.