Why would a pregnant woman drink beer?

I was a server at a steakhouse and one night I had a couple at one of my tables and when I took their drink order, they both ordered draft beer. As they ate their food they ordered several more beers each. After the girl was thinking about whether she should drink her 3rd (22 oz.) beer or not, I noticed that she was pregnant! I can refuse to serve anyone alcohol but didn't think it was my place to deny her from a moral standpoint. I personally would not drink like that if I were pregnant but what im wondering is why she might have been drinking so much? Do some doctors say drinking a few here and there is ok? Could it be that she was an alcoholic and was avoiding alcohol withdrawal? I know pregnant women addicted to methadone are encouraged not to quit during their pregnancy because the withdrawals are supposedly much more dangerous than using the drug as prescribed. Is alcohol the same way? I have never been pregnant but I have only ever heard that drinking is very bad for the unborn child...im just curious as to why someone would openly drink alcohol while pregnant...