The reason that retail stores blackout vacation dates is to ensure that the entirety of the staff is available to assist customers. With iPhone launches being so high-profile, it would make sense for carriers to make sure that all employees are available to sell the new devices.
Interestingly, it seems that the launch of the new iPhones will be Apple’s first simultaneous release of iPhones on all four major U.S. carriers. The iPhone first launched on AT&T in 2007, on Verizon and Sprint in 2011, and on T-Mobile in early 2013.
The September 20th launch date is far from a surprise based on Apple’s most recent iPhones launches. Apple typically unveils its new iPhones at a media event on a Tuesday or Wednesday, and then the device arrives for consumers on the Friday of the following week. Based on the Apple event being held on September 10th, a September 20th release date would match the existing pattern.