They'd be pretty massive liabilities and would significantly eat into the recovery for other creditors, which could have effects on Valve's ability to issue debt, or might just mean that whenever Valve issues debt there's a clause making that debt senior to any future we-terminated-your-account liabilities.
How does that work? Surely that’d have to be something agreed to by both the lender and the customers?
If there are literally no assets there's very little that can be done though, you could threaten the owners with jail but what can they even do if there is no money? If there were a law it would almost necessitate something more like unemployment insurance where companies are forced to hold insurance to deal with this in a predictable way if they shut down or close a customers account.
Actually, unemployment insurance might be a pretty good analogy. If a company goes out of business and you get laid off as an employee you get unemployment mostly paid for out of money the company paid to an insurance policy. In this case instead of being triggered by unemployment it's triggered by loss of access. In the same way, if a company is closing a lot of accounts their insurance rates go up.