Look at it from a commercial standpoint. On a runway that requires derate in order to lift a higher load, a weather system which contaminates the runway suddenly costs the company direct revenue if derate is not allowed. With flex/toga it's a case of engine wear, so assuming the load doesn't require toga then choosing toga is safety related but doesn't affect the load.
So I believe that there has been a dispensation made to allow for better commercial viability. I guess when possible always use toga on contaminated runways, but if you need derate then alright. We can live with that as the increase in risk is acceptable.