Its a rather outdated law but thats the case for many laws in Thailand including the silly 2 - 5pm, the Election Day ban etc, surely in this day & era it should be dont drink alcohol on the day if thats how you feel, much like some Catholics dont eat meat on Fridays!!!
The irony of these silly laws is that a large percentage of Thai's including the makers of these laws will have an alcoholic drink on the day so it makes little sense, maybe in the next century they will get up to speed with reality??