#1 They usually do this to make their thousands of subscribers happy. Since some people live overseas, or move and have their mail forwarded, the advanced date sometimes arrives "on time" for these people. I've read this explanation in several magazines over the years. IMHO though, I really believe it is for option #2.
#2 Also, think about the automotive industry. I want an '06 model now, not an '05. The same goes for publications... people feel that they are getting the latest and greatest when they are buying something dated a month in advance.