A little harder to enforce ('no, judge, it says SHOULD, grant our civil suit please') and a little easier to grant exceptions for than saying "must" (ah sorry customers, bigco had a bunch of money, and the rules don't say MUST). It's a well known wording choice in software, see every RFC ever.

It signals to me that Valve wanted an out for their rules when they wrote them because it's an odd choice for such a well known word choice otherwise.