Well you can't call it common without generalizing quite a large group of very friendly and helpful people, but yes there are both bad guys and good guys out there.
Many people don't know or find it hard to create a passworded private server so they use a public server instead. To prevent others from disrupting their action they tend to throw anyone out that doesn't belong to the game. There are some signs which you can use to avoid problems though:
- Usually when people play 1v1 they do this on purpose, try to join at least 2v2 (or more player) games.
- When two clans play against each other they usually don't like interruption from non-members. Try to join games with random people instead.
- Try to find some good servers and save those as favorites. Then you can use these as regular servers where you can always come and play.
Hope this will help.