As opposed to the crowd sobering up and leaving.
But also I believe the bar (staff) often genuinely don't know what their serving is harmful.
I should have added the limit to safety at low levels of methanol is actually that your body processes ethanol much faster than methanol. So it's more that the crowd goes home and then hours later (once ethanol has been cleared) the methanol finally is picked up by the enzymes that makes it toxic. If they stay drunk (on ethanol) for days the methanol might have been excreted before being toxified.
So maybe the answer is water down the shots of your obviously drunk customers.
TBH, I also had to do my own bit of googling because I barely drink alcohol to begin with, but it does look like "at the start", it's not very distinguishable from ethanol in taste and in effect.
Whatever subtle differences exist between them are probably unnoticeable to people who are already drunk, not to mention drinking cocktails with all sorts of other flavors mixed in.