A small 6% pour is a bit ludicrous.
But when it's 9% or more? Makes more sense.
Especially if it's well made, someone could knock back a few pints of a heavy hitter before they know what hits them, and that's a potential issue for an establishment. A few years ago I was brewing for a place that had a sneaky very strong beer. After a few too many people had a few too many glasses (to the point where cops had to be called), they opted to cut off strong beers at two glasses for the same reason. But smaller pours allows both patrons AND bartenders to meter it since it slows consumption.
A few beer bars around here also do rare (expensive) beers in smaller pours regardless of strength. Allows more people to get some, and prevents a stupidly expensive full glass.
If it's clearly marked (very rarely have I seen it not) that it's a smaller glass, then it's ultimately on you for not paying attention. Too small a pour? Don't order it.