Not hard, and they did it a long time ago. The user didn't opt in.
On top of that the user
1) Chose to turn off (or not use) malware software
2) Enabled Java in the browser
3) Chose to visit a short url link presented in a chat window
4) Clicked through a big scary warning
All while still logged into their MtGox account.
It sucks that they're a victim of crime, but their actions were dumb.