When a company envisions proprietary relicensing as a business model for themselves and doesn't want to compete with other contributors to the codebase in a proprietary relicensing business landscape that company will chose an (A/L)GPL licensing model with copyright assignment to keep other entities for competing for the same business on equal terms.
When codebases are BSD license, copyright assignment doesn't create any special relicensing privileges for any contributor or contributing entity. When its BSD code, anyone can take the code a make a proprietary fork and compete with everyone else for the same market.
Mark Shuttleworth continues to over simplify the issues. The issue is not copyright assignment. The issue is the interaction of copyright assignment with copyright licensing choices in a way that deliberatively creates an unfair business advantage for one contributor over all others. He wants to paint opponents as a fundamentally oppose to assignment because he needs to point opponents as unreasonable in order for his point of view to seem relatively more reasonable. This is naked rhetorical manipulation of the discussion. At this point, he should know better. It's time to stop manipulate perception and to start having a conversation on the merits.