That's for the retail release. AT&T did the same thing. However, preorders will be fulfilled. I confirmed that my new Galaxy S3 is waiting for me when I get home.
I don't think the carriers expected this much of a demand, but it's not that much different from the first iPhone releases. There were plenty of people turned away after stock was depleted.
*edit*
Are you saying your preorder was pushed back too?
*edit 2*
Engadget says Sprint preorders not affected.
http://www.engadget.com/2012/06/20/sprint-galaxy-s-iii-delay/