The reason for this is quite simple. Big Red is one of the companies involved with Google Wallet competitor ISIS, which also uses NFC technology. Others involved with ISIS include manufacturers HTC, LG, Motorola, Sony Ericsson, RIM and Samsung. Other carriers involved are AT&T and T-Mobile. Interestingly, both of those mobile operators offer carrier-branded versions of the Nexus S with NFC capabilities, but like Verizon will allegedly do with the GALAXY Nexus, Google Wallet is blocked from use on those models by each carrier.
So now, we have the ironic situation of a Google app unable to work on a pure Google phone. With AT&T and T-Mobile part of the ISIS family, we wouldn't be too surprised if Google Wallet turns up missing on a future GSM model of the Samsung GALAXY Nexus for those two carriers as well.
Unless Google wants only Sprint to offer phones that can access Google Wallet, some of those involved with ISIS are going to have to start being a little more flexible toward Google Wallet.
source: 9to5google via AndroidCentral