You have to imagine that if they go after Valve on those kinds of grounds (ie liability due to indirectly profiting from third party actions), they would need to have an air tight case or else it will almost certainly end up in court. Which I'm sure is something that Valve does not want.
My understanding of the way most of the gambling sites operated is that they used the trading system and never used the Steam Marketplace (for reference I used one of the Dota 2 item betting sites several years ago - I am of legal gambling age before anyone asks). Maybe the CS:GO stuff is different though, I'm not sure. If Valve could somehow definitively say these third parties never used the marketplace to conduct these things (this seems doubtful, but I have no idea what kinds of logs are kept on Valve's side), then there would be no grounds for the commission to recoup money since Valve never profited. But without being able to definitively say what has occured -- Valve's letter even admits that it happens outside of their knowledge most of the time -- then it feels like really murky waters to me. The kind of waters that a state comission looking for money might try to test in court if they see big $$$ in it for them (which it seems like they do in this case).
The next developments will remain interesting if nothing else. Some tighter regulation on the gambling side of the video game industry might not be a bad thing overall for the players. Especially as regards mobile games but, sadly IMO, none of that is at issue here.
The issue with marketplace transactions as it pertains to gambling is it gives users an outlet to "cash out" their winnings into credit that can then be used to purchase additional items to place bets with, or to exchange for video game purchases. Obviously this is especially true of items who's market value is less than $500 (the limit Valve places on wallet funds). In addition to Valve getting a cut of that transaction when the item is sold, they're also receiving additional financial benefit because the credit is locked into their ecosystem. If a user decides to "cash out their winnings" and then spends that wallet money on a video game, Valve gets an additional 30% of that transaction. So as I said before, even though they're not directly engaged in any illegal activity they are certainly profiting from people using their service in unintended ways.
I wouldn't think that Valve would want to get into any legal scuffles with the state gambling commission, as that might bring other practices of theirs to light - such as the gambling system that is built into and operating on Valve's own system in the form of crate / key gachapon in various games where Valve controls an item's rarity and thus it's marketplace value. Again, this could be seen as gambling under state laws, and would be further exacerbated for the same reasons as above (namely, that Valve profits greatly from this, is targeted towards minors, etc). Some people would argue against this, saying it's no different from mobile F2P games or even Magic: The Gathering cards, but I think it's a fundamental shift that the high-value items you're trying to get from the gacha hold an intrinsic value that can be cashed out for credit by selling it to other players. These practices are expressly forbidden by the state's gambling laws, but Valve feels like they don't apply to them because of reasons.
TL;DR: The next developments will certainly be interesting. Valve could potentially face huge fines (sanctions) if they choose not to comply, which upon non-payment could result in court battles and/or arrests. Valve has always been at the forefront of fair consumer practices in gaming, so it's disappointing to me personally to see them take such a nonchalant stance towards these gambling sites. Either way, I don't think we've heard the last of this issue.
