Most of the people who participate in Beta testing are nice, kind, courteous people who understand the importance of complimenting others and supporting companies. They're the sort of people who fill out surveys and what not because it's the right thing to do. Unfortunately, these types of people are on average not-particularly-great at strategy games, which requires a different and less pleasant set of skills.
The wolves and the jerks only come out afterwards, when the game is released because they don't want to pay for a game until, you know, it's been tested by other people (let them be the guinea pigs).
Unfortunately, it's the wolves who have the expertise in breaking game mechanics. As a result during Beta Testing the developers are constantly being complimented about how they managed to get the game to work. It's only AFTER beta testing that real testing of the quality of the game begins. And THAT is when the jerks come out (unfortunately many of the jerks are also the ones who understand game mechanics the best).