TB20 pilot reports being issued a low altitude alert at 2;500 FT during the RNAV Z Runway 28L approach into HWD. When the reporter advises that he is on the approach path the Controller advises; 'That's normal for the RNAV Z approach; but we have to give the warning.'

Narrative

I was on the HWD RNAV Z [Runway] 28L approach (an LPV approach) with needles centered. Weather was solid IMC from 6;500 MSL down to around 600 MSL. At approximately 2;500 FT; the HWD Tower Controller said; 'Caution; low altitude alert! Check your altitude!' This caused a moment of panic and distraction as I double checked the instruments and approach plate. I replied; 'We're showing our path to be right on glideslope.' The Controller replied; 'Yeah; that's normal for the RNAV Z approach; but we have to give the warning.' Although apparently mandated by the rules; this warning was completely unwarranted in this case. It was also very unnerving and extremely distracting during a critical phase of flight; which could cause an accident. This is; of course; exactly the opposite of what the warning is designed to do. Furthermore; unwarranted warnings like this could cause pilots to become 'jaded' and less responsive to warnings that are actually important - like with 'the little boy who cried wolf.' In my view; warnings like this should never be given to pilots on instrument approaches; when the aircraft is within the tolerance of the approach path.