It could well have emerged as a cultural norm from the prevalence of heads, but I've seen it often for very moderate heads, with a gap left above the foam.

> not cheating the customer out of volume

I don't think it's cheating if its the norm. One would expect prices to be set appropriately for the average volume served (i.e. a full glass would be a bonus rather than the gap being a loss).

I do just find it odd, coming myself from the opposite culturally.