It's common for open source libraries that use split licencing. By assigning copywrite ownership to the project, they are able to licence your contribution howe

It's common for open source libraries that use split licencing. By assigning copywrite ownership to the project, they are able to licence your contribution however they like. Without a CLA, any licence change, sub-licencing or commercial terms would need to be negotated with every individual that has contributed code to the project