It kind of has to be legal, because otherwise Steam would be declaring war on all sales-on-other-platforms-but-not-on-Steam, including the popular megabundles, even if they don't provide Steam keys - which would be nuts. They have an obvious and legitimate interest in discouraging people from buying Steam's own services for cheaper than the way that gets Steam money, but not in restricting sale of non-Steam products, i.e. the games themselves.
One cannot always trust lawyers to be reasonable. However, you probably can trust it if you've emailed them saying "this is my understanding of the contract, tell me within two business weeks if it's wrong" and gotten no response within a fortnight. (I am not actually a lawyer, and my having-of-legal-opinions is not endorsed by an academy of law, nor am I in a position to pay reparations if you get sued and the courts turn out to be spectacularly corrupt my advice leads you astray. Heed it at your own risk, small though I judge that potential risk to be.)
It might go better to call the keyless-version-at-current-price a bundle sale that you just happen to run most or all of the time?