I would say nowadays iPhone 4S selling is not in as high as we saw because iPhone 5 will come next month. So it could be the anticipation of people from iPhone 5.
But still the price of $149 is applied in other US carriers like AT&T and Verizon. According to me, it is a better opportunity for Sprint to showcase their business in front of people and it will helps them to give a tough competition to their rival carriers like AT&T and Verizon.
The drop of $50 is accompanied by the waiver of the $36 activation fee as well, so this should be the cheapest way for Sprint customers to get the latest version of the iPhone while it’s still the latest version.