There's probably pragmatic reasons for it. Imagine this...
"Oh dear, I had no idea this drink had alcohol in it... (wink, wink)... those naughty, naughty laypeople giving me inadmissable offerings (wink, wink, guffaw, hic)"
If it's a Vinaya offence regardless, it cuts out the wink winks and encourages monks to be vigilant about such offerings.
As an aside, the impact of lack of sleep on mindfulness (think about road safety advertising, for example) is often said to exceed that of a small amount of alcohol. I'm sure you've been less mindful before at 2am than after a 0.5% beer during the day.... and mindfulness is whole the point, yes?
I would see sila as a means to an end, not an end in itself.