and bought the iPhone at the $600 price Why would Apple offer rebates to customers who already gave them money and bought the iphone at the $600 price?
I could guess it has to do with brand loyalty and the rebates being an attempt to show sympathy and trustworthiness, therefore simultaneously winning the hearts of on-the-fence consumers and also wining back apple fans who got burned by the 200 dollar price drop.
Could there be any other reasons?

Thanks for your help, all.

No, you pretty much nailed it.
Since people buy new cell phones after 1-2 years, it will keep Apple in good favor of those who bought the iphone, thus, increasing the chances those people will continue to replace their old iphones with new iphones.