As part of the agreement that allows Verizon to exclusively sell the Pixel and Pixel XL in the United States, Google has bizarrely relinquished control of delivering Android software updates to Verizon.
In a move that truly highlights the death of the Nexus program, system updates for the Pixel phones will be managed by Verizon for models that it sells, while Google will handle updates for unlocked models purchased through the Google Store. Phones bought at Best Buy are Verizon models, so they will be subject to Verizon's update schedule too.
With Nexus devices, Google controlled the delivery of software updates, whether it was sold by a carrier or not. This ensured that anyone who purchased a Nexus device was treated to fast software updates when a new version of Android was released. However, with Verizon controlling updates for its Pixel phones, the carrier will almost certainly delay the rollout of updates to ensure their bloatware is fully compatible.
On a positive note, Google will still push monthly security updates to the Pixel and Pixel XL outside of Verizon's control, ensuring users have a secure device at all times. Verizon only has control over system updates, such as a potential future update to Android 8.0 when it's announced and released.
For those that want a Pixel phone on Verizon and don't want to deal with the carrier's slow updates and bloatware, the models Google are selling through the Google Store are fully compatible with the Verizon network. It's possible to simply buy the handset from Google, rather than Verizon, and enjoy a Nexus-like software experience.