Last July, Valve began blocking the CSGO gambling accounts, after making their position on gambling websites clear
Recently, some gambling websites have begun using Team Fortress 2 items as bargaining chips, prompting Valve to start blocking TF2 gambling accounts as well.
"In July of last year we outlined our position on gambling web sites, specifically noting that Valve has no business relationship with these sites. At that time we also began blocking many CSGO gambling accounts.
More recently, some gambling web sites started leveraging TF2 items. Today we began the process of blocking TF2 gambling accounts as well. We recommend you don’t trade with these sites."