Van drivers remember the airlines and crews who don't tip.
So, next time your crew has been sitting on the curb for 20 minutes waiting for the van, stewing in your discontent about YOUR wasted time at the curb, consider the cost to you that welching on one dollar tip each way has...especially when the van driver pulls up and doesn't so much as acknowledge the wait. Why should he bust his hump for you to earn a tip when he knows you wouldn't give one anyway?
Again, at most regionals you can earn $2 in per-diem while sleeping for less than 90 minutes...and most of you shysters claiming broke won't think twice about tipping the hotel or restaurant bartender who does nothing but take the tops of your longnecks while on that same overnight.
If you're jealous of the money you THINK van drivers make and that's why you don't tip them, then quit flying and become a van driver. Otherwise, if the van is waiting for you, the driver is safe and they take care to load/unload your bags, drop the Washington on them and make your captain buy your beer on the RON or your coffee in the morning.