Those banned held accounts for Diablo II and Warcraft III, and the hammer more than likely fell due to hacks and third-party programs which Blizzard said harms the stability of its servers.
"If this is a first offense, the CD key associated with the banned account will be suspended for 30 days, while repeat offenders will see their keys banned permanently. All account ban decisions are final," read the post.
"We would like all players to remember that abuse of unintended mechanics and/or use of third party programs is a violation of the agreement made when signing on to Battle.net, and can subject your account to disciplinary action up to and including a permanent ban of its access to the service. These types of activities can severely impact the stability of our servers, and we’ll continue to aggressively monitor Battle.net in order to protect the service and its players from the harmful effects of cheating.
"Many account closures come as the direct result of tips emailed to our hacks team by legitimate Battle.net users. If you come across a hack, find a site responsible for distributing hacks, or have a replay of a newly available hack, please report this to our hacks team."
Somewhere round the 300K mark seems to be a good number for Blizzard, as 350K accounts were banned for the exact same reason in November 2008.
Keep your noses clean then folks, otherwise no StarCraft II or Diablo III for you - should either finally see the light of day.