That's so true!
I
think they do
this on the assumption that once they've installed a battery with whatever charge it has in it (regardless of whether that's just the dribble left over from the 'thoroughly cursory quick zap' of a charge that it got when it was made, or maybe they were conscientous enough to give it a couple of hours of charging before installing it, but they rarely give them the
minimum of 8+ hours of 'proper charging' that they really need in order to be truly 'initialised and fully charged' to start with!), then you riding it home will be a long enough ride to charge the battery up sufficiently to negate the need for them to do that initial full charge, apparently not realising (or maybe they
DO know, but are just ignoring that cos they think
you DON'T know?! 
) that
the vast majority of motor vehicle alternators ARE NOT DESIGNED TO, NOR ARE THEY TRULY CAPABLE OF, 'FULLY' CHARGING A NEVER CHARGED &/or a NEW/DEEPLY DISCHARGED BATTERY!!
Most alternators are only designed & capable of 'topping up' the charge in a battery to usable levels and then maintaining that level, but most won't actually ever '
Fully' charge your battery, and they generally take at least 30 minutes of riding at max alternator output revs just to replace the charge depleted by ONE start (btw, that's 30 mins for 'normal' cars, it's somewhat more for our extremely power hungry Spyders!) and that thing about giving your new battery a '
FULL' charge before setting it to work is an extremely important/essential part of setting the battery up so that you stand ANY chance of getting anywhere near close to it's 'normal life expectancy' - you don't do it, your battery
WILL NOT last as long as it could, meaning you'll be forking out for a new one again, much sooner than strictly necessary!!
Hmm, hang on, who's likely to get more business/greater profits by doing this?!

Could it possibly be a deliberate decision on the dealer's behalf??