The only people that have a published height limitation are commercial operators working under Part 333 and 107. The amateurs and hobbyists are simply required to maintain safe separation from manned aircraft and to operate safely. Everyone is subject to the "careless and reckless operation of an aircraft" provisions in aviation law but a mandated 400' altitude limit, be it listed as MSL or AGL, is not part of the program.
The so called 400' limit is not a limit at all, nor is it a law or regulation referenced in Title 49 of the CFR's. In no location of the FAR's is a 400' hard limit placed on sUAS or model aviation that blankets the U.S. You are however, limited to operating within line of sight (LOS) as referenced in Part 336 of the CFR's.