Legally, there is no difference between "fansubs" and "bootlegs". In the eyes of the law, both could be seen as damaging to the market. Regardless of whether or not a title has been technicaly licensed in North America, it is illegal. Lack of enforsement of copyright laws in terms of unlicensed fansubs maybe the result of several different factors. Some companies may believe that the early introduction of the title to North America is beneficial. Others may simply tolerate a "fan-activity" as long as it does not become too damaging to sales. And yet other companies may not want to or be able to invest the time and money necessary to prosecute foreign violations of their copyright.
In the end, regardless of ethics, or motive, fansubs are technicaly illegal.