Now we're conflating two issues: contributor agreement != copyright assignment. I was considering the case where if a project is GPL and one person/company owns 95% of the code base, then requiring contributions to be BSD doesn't appear to have any benefits over copyright assignment.
There are contributor agreements that don't do copyright assignment, Google's for example. They just want a statement that you own the code and won't assert any patents.
But there's your statement: "... in a way that deliberatively creates an unfair business advantage for one contributor over all others". Besides that being the whole point of copyright, I have a hard time seeing this as automatically unreasonable. If some company is investing in a project, why shouldn't they get some benefits? As long as all contributions are available in an open source release, you still get to fork if you really want to. What's the catch I'm missing?