I'm wondering if this is due to the prevalence of cask ales vs bottle/keg conditioning. The former is relatively uncommon in Belgium and you want the head from the latter.
That said, oversized glassware (e.g. Duvel's tulip for aromatics) and/or fill lines are also used to accommodate the head while still not cheating the customer out of volume.
> 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.
The problem was that many people insisted on the glass being filled to the brim, because they felt they were being short changed. So it solved one problem but created another.
In Spain they try to create the head by dumping the beer in the glass, Guinness style, and letting it excessively foam up. So it's half flat and half full.
True but a mlíko pour is a special request and usually cheaper.
Guinness glasses are exactly a pint, so the Guinness head means you're getting less than a pint of actual beer.
This is tolerated/expected and so de facto correct but de jure perhaps not.
I hate to be pedantic but pint being volumetric, you're still getting a pint, independent of density. Also - a nitrogen head doesn't dissipate, so you never get a gap.
I'm now curious though whether a nitrogen head is less dense than a CO2 head...