Two points to consider here:
1 - Perceived value of the WF assets.
2 - Synergy.

1 - Clearly, Amazon believes they can leverage WFs assets (B&M storefronts) much more effectively than the current management regime.  Based on regional location, they can serve as hubs for a distribution network.

2 - On the topic of distribution networks, that's a core competency for Amazon.  So, will they diversify the product offerings in current stores to include more than what is currently sold there?  Possibly... a Kindle Fire with those marshmallows?

Make no mistake though, this is a deal focused on US opportunities.  Canadian WF stores are residual, and will probably be assessed separately.  Expect any range of decisions regarding the Canadian assets, from attempting to deploy the same model envisioned in the U.S., to shuttering or selling them off.  

As Target found, dynamics are different up here in the GWN.  You don't need to look farther than amazon.ca to see that.