Derek...
That's Bosch protecting itself against people who fuck up a dual battery
installation and want a free starter after they ruin one.
The resistance thru the starter motor isn't going to change, so at 12.5V
the amperage is going to have a maximum.  The motor is designed to handle
this maximum and you cannot accidently exceed it just by jumping more
batteries to it (in parallel...).
Bosch is just protecting itself against those who hook 'em up in 
series.  :)
Sizzle!

Cheers

<sstones@io.org>
SStones             Toronto,  Ontario.