. . .which government says "as much as you want"?
Interesting point taxy, but I believe the only way I've ever seen is by volume.
Also, seeing as beer (+wine, hard liquor, etc) is a liquid and sold by liquid measure it doesn't make sense to list by weight.
In Canada, the guide for alcohol consumption is:-
light beer of 3.7% = a 0.7 standard drink
Per week - fewer than 10 standard drinks or 15 for men.
Do the math!
If you are looking for the optimum consumption of alcohol for health - a lot of people would tell you that 0% is probably best. That being said, the above guidelines are probably pretty standard, give or take a bit, for most countries that have investigated alcohol related problems.
By weight is used in Germany and probably most of Europe. I think it doesn't make sense to measure it by volume. If you take 30 ml of alcohol and add it to 70 ml of water, you will get less than 100 ml of mixture. Water is "fully miscible" in alcohol. The molecules fit in between the alcohol molecules in a way that makes the resulting volume less.
By volume is used in most of the US and Canada. I knew a German brew master who was hired to come to Quebec and work for one of the local craft breweries. The SAQ told him that his beer was stronger than he claimed. Then, he found out that we use ABV in Quebec. His beer was, on measuring the alcohol, exactly what he said it was, but by weight.
OK then in a conclusion, is it okay to drink 3.6% vol Alcohol by volume for 5 can a week (each approx. 500ml). I cannot live without it but I am good at controlling myself