why do u realy think they do !!
i guess they want there device to be slow and slagy
as for the statment " Our devices have the ability to be stable at 900+MHz. " .. yes you have flashed the 900mhz kernel and yes the cpu SOMETIMES hit that speed , and still works .
the question is FOR how much can it run at 900mhz without causing any real problems ? to the cpu or any of the other circuits in there ..
remember various components are in a very tight space and with no air vents of any kind , so overheating may be an issue here.