My guess is it's got one of those "kickstand" type dealies and is meant to support up to 1,000lbs of force, not 1,000lbs of fat. No clue what the equations are for dropping a lardbeast onto a surface, but runners typically put 2-3x their bodyweight on each knee at every step. We'll go with 2x just to keep things simple for our purposes. That means if the toilet can withstand 1,000lbs of force, it will likely break after a 500lbs hambeast plops down on it. Possibly a 400lb one as well, depending on how hard they drop. Sure, if they sat down carefully, it would be fine. But at 500lbs, someone can barely walk, meaning they're going to just drop their bucket of lard down onto the seat with a grunt.  So while the toilet can support "up to 1,000lbs", an obeast of 400lbs or more is likely to exert more than 1,000lbs of force on that poor device when they go to defile the restroom.