I think that's fine, and in fact I wish -all- benchmark programs had to follow that rule. I have written a lot of test/exercise code, and I know how easy it is to have unintentional bias leak in.
Also, releasing source of the benchmark means that -all- the tested vendors can tweak their code for better results, instead of only the vendor who pays off the benchmark writers. [slight /sarc] The source release requirement can only improve the quality of the benchmark.
I trust proprietary benchmark programs even less than I trust proprietary applications. I use proprietary software when I have to, and I rely on it to a degree every day, but never as much as if it were open.
It may be nice practice, but it shouldn't be forced. Here the king of proprietary software is telling everybody else to release their code. I don't think license terms for such products should be able to affect your freedom of speech. Boiled down, benchmarks are just "I used this publicly available program and here are my experiences." Courts aren't likely to allow such speech to be restricted or subject to terms, and so far they haven't.
This is of course separate from a private alpha or beta program where you sign an NDA for the privilege of being in on the program, and such NDAs have expirations (usually at the end of the program).