It's common for many types of cells, but much more difficult for Sunpower cells. Instead of having top contacts, Sunpower cells have a fine array of contacts on the bottom [1]. Cutting this without creating shorts or defects is quite difficult. It would be better to have Sunpower custom make them.

Given SpaceX deadlines it's possible that they just ran out of time for this optimization.

[1] https://sc03.alicdn.com/kf/HTB1XRXrHVXXXXXeaXXXq6xXFXXXM/3-5...