I read the following in ØMQ - The Guide (in Chapter 6, in the section Licensing and Ownership):
All patches are owned by their authors. There SHALL NOT be any copyright assignment process.
Here we come to the key reason people trust their investments in ZeroMQ: it's logistically impossible to buy the copyrights to create a closed source competitor to ZeroMQ. iMatix can't do this either. And the more people that send patches, the harder it becomes. ZeroMQ isn't just free and open today—this specific rule means it will remain so forever. Note that it's not the case in all GPL projects, many of which still ask for copyright transfer back to the maintainers.
I don't understand the last sentence: how can a GPL project ask for copyright transfer back to the maintainers?
Can you give any example[s] of [a] GPL project[s] which do[es] that?
What is a contributor's incentive to make such a copyright transfer to "the maintainers"?
Is this topic (a requirement for copyright assignment) inevitably caused by the maintainers wanting to dual-license the software?