Venting is required to mitigate the risk of accidental explosions (see pp. 14-15, particularly pp. 15 in the footnotes). Presumably there are fewer knock-on effects of an accidental explosion outside the parking orbit, though yeah, I'm also not quite clear why it needs to be vented prior to getting there.
Dirtward is no-go because of how fucking high geostationary orbit is. Way easier (in terms of delta-v) to boost it up 300 km than changing the perigee to something < 500 km (roughly the height of SpaceX's current Starlink sats, which will deorbit and burn in a few years at that altitude, thus mitigating space junk risk) from 35,786 km (geo orbit).
Edit: venting propellant prior to moving to parking orbit would mean less delta-v required to move the thing to parking orbit in the first place (less mass to move), so that's my educated guess...they might not have enough delta-v to get there without venting first.