Politeness is maintained on the mailing list, but undesirable posters are tracked down and bullied behind the scenes where proof who's doing it is difficult. This happened to me 10 yrs ago when I started posting casually on a mailing list for a new programming language. I've found overall (not by statistical analysis, but by personal impression) that this tends to happen more when the project leader has a management rather than technical background. Many open source contributors learn their skills in paid employment first so the political skills of online project managers are often more developed than what you find on the job. And because open source contribution is far more global and with a more permanent record than a local job somewhere, the effects are often long-lasting for those involved.
In such good cop bad cop behavior, maybe the real situation tends to show itself over time, just as shown in the study. That project I was trying to politely join has evolved into a blatant "smoke and mirrors" environment that few people trust.