Apple hopes that it leads to more songs being sold (I would think that would depend on the band, a song might sound great for half a minute but really lose momentum at 90 seconds). But what's interesting here is the way they're handling this change. Engadget has a copy of the notice sent out to (whatever smoldering remains still exist of) record labels and the letter says, basically, we're doing this, it's for your own good, call us if it's a problem. As the world's largest music retailer, Apple can make these moves unilaterally.
This comes at a time when online music sales are still strong but more leveled off than before.