I may take you up on that offer...
And why are they alternative instead of rock? Well as both categories are very open to interpretation, let's look at the other nominees in both categories:
Their "rock" category focuses more on long time, radio worn artists (Daughtry and Wilco being exceptions in that they are not long existing and radio worn respectfully). Their "alternative" category includes mostly bands that appeal more to a cult fanbase (The White Stripes and The Shins are also exceptions, in that both have had their albums this year go to #2). Overall, it's hard to tell exactly what makes one rock and one alternative.