He showcased the vulnerability by making the screen shake and play the Harlem Shake. Valve banned him shortly after for "violations of the Steam Subscriber Agreement." The issue here is that according to Duda, he tried to inform Valve of the vulnerability multiple times prior to this move on his part. His warning and explanation of the security flaw apparently fell on deaf ears multiple times.
This is completely absurd on Valve's part, especially if the claims from Duda where he tried to inform them of this are indeed true. This is a means of bringing attention to a major security issue that should have never been an issue in the first place if Valve were vigilant or even if Valve listened to the warnings.
Also, why use your business account to show the flaw? Maybe you have to have a special privilege to do this? If not, you create a fake account and post it up. You wont be a hero for showing an exploit that everyone tried to ignore. It happens all the time with even people being prosecuted.
Oh well. Hopefully Valve learned something, but the ban doesn't really show that.