Because a long time ago in a galaxy far far away I signed up with Sprint using a credit union discount that gives me 25% off my bill. When the S3 came out and people renewed their contracts Sprint took a look at their plans and forced them to submit paperwork to prove the discount. Well, nobody had the right credentials to get that type of discount anymore so they lost out. I'm worried buying the phone through Sprint will tip them off to my discount.
(I've successfully transferred service to my current S3 that I had bought off ebay without a hitch).
Plus I think buying the phone outright and activating it myself online would save on the $36 activation fee as well.