Being in full manual, it doesn't make a whole lot of sense to need to lock an exposure in, say lock in ISO at 1600 instead of moving over to a darker subject and it metering at ISO 3200. However, it would still make sense to be able to at least have the option.
Full manual with Auto ISO is no longer 'full manual'.
It is like Av or Tv with Auto ISO except you control 1 more of the exposure triangle yourself - its still a metered mode. Shame it doesnt have exposure compensation