Why not though? The entirety of the LLVM project is available to them, and you, for free, as is the RISC-V ISA itself. A lot of people are getting a lot of value from free and open software, and they may feel their contributions are in a like spirit.

Folks that do this work for "free" do it because they enjoy it.

And a small observation: if you require money to do something, you usually have no chance of being as good as the folks that do it for the pleasure.