For several years now, high school, college, and professional sports has been struggling with scandal after scandal due to the criminal behaviors of the athletes. We've had reports of gang activity, shootings, rampant drugs, thug behavior, and now the Vick stuff. There are property crimes, murder, and entourages involved in crimes. The colleges and pros especially have tried to impose controls through demanding a dress code, drug tests, and the like. Mostly, ineffectively.
If the NFL bans Vick for life, it creates the pattern and precedent needed to ban other criminals. The NFL - and other pro sports and college teams - have a vested interest in protecting their image and brand. By welcoming Darryl Strawberry back over and over again, pro sports created the situation in which athletes know that their skills and ability to make money for the team owners is more important than their status as role model or the image they project for the team or league.
Banning Vick will be a first step in the leagues regaining control over criminal behavior from players (0r other affiliated persons). To say nothing of being the "right thing" to do.