As Lescastor said, some GMs ban first and investigate later (or not at all). Plus if people report every little thing, many of which can be easily solved with a simple /tell or "that's inappropriate" in chat, then the GMs will have even more reports to deal with; and since the work day hasn't gotten any longer, that means less time with each report, less time to see all the facts, less time to get the whole story, and more snap decision bans of players who only really only needed a warning or infraction point.
Where if you only report the bad things, it will (in theory) leave more time for the GMs to investigate those cases and give appropriate judgments. I know I'd rather see someone get a longer (or permanent) ban for something really bad (after a more detailed investigation) than I would like to see them (with their really bad incident) and 20 people with minor incidents get short bans/infraction points because the GM doesn't have time to look into the incidents in detail.