There are two things that lead to the controversy: 1) They are re-licensing a formerly open source project, turning it into a closed source project. I am not

There are two things that lead to the controversy: 1) They are re-licensing a formerly open source project, turning it into a closed source project. I am not familiar with their community, but there is a good chance that they have attracted users and possibly contributors based on the fact that they the software was open source. While these folks can continue to use the old version, such a betrayal of the community cannot be expected to be taken lightly. 2) The product relies on and is built on top of open source software (Kafka). As it is a part of the Kafka ecosystem, it can bee seen as a betrayal to the entire ecosystem to change course like this. So it is certainly within their legal rights to close their source like this, and it is within the communities rights, both legal and moral, to decry the change.