Although the two executives were not personally implicated, the company had little choice than to accept their resignations and make a public disclosure about what happened.
Read all about it:
An online-fraud scandal in China: Alibaba and the 2,236 thieves
The Economist (check out this December 2010 article as well)
Why Alibaba's CEO had to go
Alibaba.com CEO Resigns in Wake of Fraud by Sellers
David Wei and Elvis Lee Quit Alibaba Amid Fraud Inquiry
I think this is only the tip of an iceberg ... watch it closely.