There's several qualifiers I have with what you're saying, but I'll start by saying that your sentiment is completely true. Yes, Riot does seem like they ban for chat offenses far more easily than trolling or intentionally feeding.
First, chat offenses are orders of magnitude easier to detect than intentionally feeding or trolling. Thus, chat offenses are punished more quickly than other means.
Second, players overestimate how often they see a troll or intentional feeder. Toxic players in particular VASTLY overestimate how many trolls and feeders they encounter. Chances are, if a toxic player claims they saw an intentional feeder or a troll, they're full of shit. Obviously, some people are trolling or intentionally feeding. However, I would not trust someone banned for chat reasons to be the person assessing it.