Yeah, they really try to get you with that. I went to Cobb's comedy club in San Francisco and they had a two drink minimum for everyone. Drinks are already crazy expensive in the city plus, I went with my girl. Those four drinks added $50 added $50 ontonthe cost of going there.
I've always wondered this question as well. Same goes for minimums to use credit cards at bars. I seen certain places that has a $10 minimum to use your credit card and others that have a $40 minimum.
The real answer, is that it depends on what state you're in. I remember when I was in college, bars with try to enforce minimums when in fact it was illegal according to city laws.
The other question you have to ask is, can you really get away with calling them out. Most bars reserve the right to deny entry or kick you out for whatever BS reason they make up.