I agree that it is arbitrary, and yet it does seem that food truck workers are working harder and under worse conditions than plain fast food.
A better reason, more self-interested, is that the food trucks are very convenient to me and I hope they continue showing up to my condo complex. I frequent them and tip well in the hopes that it will entice them to come back. The fast food places I frequent don’t require such enticement.
And as a purely practical matter, In-n-Out doesn’t have any means of tipping at the drive thru. I could probably find some workaround, but they certainly aren’t encouraging tipping.
Servers in a full service restaurant are normally paid below minimum wage, according to state laws because they are receiving a large portion of their wage in the form of tips.
Cooks, clean-up staff, etc. are required to be paid the minimum wage.
I haven’t seen that in reference to “tip pooling” although I’ve seen restaurants that have tried to eliminate tips altogether and raise prices ( sometimes called “service included”) say that they have done that to be able to increase back of the house wages.