I dont know of any airliner where the MTOW is the same as the MLW. The Fuel Jettison system is there to comply with FAR/JAR Approach / Landing Climb regulations. As you said, on some aircraft its an option which you have to purchase, before doing so you should assess if there is an impact on your takeoff weights due to the inability to land within 15 minutes after takeoff.
The B777 system will automatically set fuel to remain to the MLW, however this can be adjusted by the FUEL TO REMAIN selector. It isnt a limitation!
Now who exactly is lying to the engineers????
Mutt