After about 13 years off and on, of messing with the B737, interesting questions pop up which get one thinking. At least for some of us; (I can't really understand the need to respond without actually providing any input, but I guess some people feel the need...
)
To answer the first part-
1. An alternate book of words (Bulfer- cockpit companion) mentions that these give a volts reading only, as they are not a source of power. Thinking from a different point of view, I'm taking this to mean that the meter is measuring at a point where there is no load, and so no attempt is made to measure/display amps.
To measure amps requires a load.
As to the logic of the master caution, it's interesting, and I would be interested to know why from an engineering point of view, but I'm reasonably satisfied about the rationale.
You are starting the APU on battery power. It's a given that there is a significant draw, and that this is normal in this circumstance-
Battery as only source of power
Deliberate APU start
Not abnormal
PS. Don't take my responses as Gospel!