Which does not disclaim "be forked and built on top of freely", especially given that Hashicorp products (in the modern era, at least) are written in Go and MPL is a file-level license: creating a new file in the same module under a non-MPL license and shipping it beside the MPL code as one broader unit is, I believe, fair game. But even still: MPL is a legitimate open source license that just asks that you give your changes back to the community. Seems like a fair trade to me.