Making the software easier to use from a legal point of view was indeed the reason. They explained why they did this here: https://github.com/zeromq/libzmq/iss

Making the software easier to use from a legal point of view was indeed the reason. They explained why they did this here: https://github.com/zeromq/libzmq/issues/2376 https://github.com/zeromq/libzmq/issues/2376 Bottom line is that their licensing with a static linking exception was kind of weird and creating a lot of issues combining zeromq code even with other open source licenses (like Apache 2.0). Interesting to see how they gathered permission to do this from the developer community. License changes like this are usually hard to realize unless you insist on copyright transfers. But in this case they managed to do it without that. So it was a collective decision. Hard to argue with that.