One of the arguments is that there are many non-Hashicorp contributors to Terraform, not to mention the larger ecosystem, who want a say in keeping their work free. This argument is even extended to Hashicorp currently only spending less than a handful of fte's on Terraform maintenance. If you subscribe to those views, you turn the "taking a free ride" argument on its head.
I'll add that at the end of the day, a maintainer of open source needs to tend to its garden in order to receive contributions. This is a case of HashiCorp failing to do that on multiple levels (devoting too few resources, changing licenses, etc). So people leave to form new communities and that's only natural.
Those contributors work will still be free for the rest of time under MPL 2.0. The change now is that new changes will take 4 years until they become MPL 2.0, but until then those new change's source is still available for people to work with, fix bugs, and adapt to their business's needs.
