Container Keys, for those unaware, are the method used to unlock loot boxes within CS:GO, and up until now could be purchased and then resold to other players. As of now, the keys are locked to the account which purchased them, and the reason for this seems to be due to money laundering.
According to Valve, organised criminals are using the keys to launder money, by purchasing keys and then reselling them to legitimate users. In a statement on the Counter-Strike website, the company said;
Worldwide fraud networks have recently shifted to using CS:GO keys to liquidate their gains. At this point, nearly all key purchases that end up being traded or sold on the marketplace are believed to be fraud-sourced. As a result we have decided that newly purchased keys will not be tradeable [sic] or marketable.
The ban will affect legitimate users too, unfortunately, but it seems the overall consensus is the feature was seldom being used by legitimate players. The company do note that any keys purchased before the change will still be tradeable, however.
CS:GO has attracted all kinds of issues before, most notably due to the skin gambling controversy in 2016 with third party gambling sites being created and promoted by notable influencers. Ultimate, Valve found themselves in some bother with the Washington State Gambling Comission.