Defaults to. You can tell it to compile yourself, if that matters to you. I don’t see what the issue is here. Why is it a problem that it defaults to binary distribution?
> The biggest problem is that the defaulting to binary distribution also means that building from source is unsupported.
I don't think it means that at all. They're both clearly supported by homebrew — one just happens to be the default. You can always use -s or --build-from-source to build whatever package you are installing.
It's running the same compilation steps that the binaries are produced from. If you can get a binary through homebrew, you can compile it from source. It's just unsupported in the sense that the developers don't want to spend their time chasing down build issues on individual user's systems. You're on your own if you break your local build environment.
(Ignoring the packages that are just wrappers around closed source software and fetch binaries whether you compile from source or not. Those are the exception, not the rule.)