Toxicity in chat is very easily detected and prove as it is right there in the chat logs, and there's no reason for you to be rude towards your teammates. Intentionally feeding/giving up in ranked games (or any game mode for that matter) is more difficult to prove because it can easily look like someone having a bad game.
Now yes, I do understand that there are clear cut case of intentionally feeding (going 0/20/0 and buying nothing but boots), but I'd be willing to bet that in most cases, the intentional feeding isn't as clear cut as this. Riot wants to be absolutely certain that they don't have any false positives, or at least have as few as possible, which is why it can take longer for intentional feeders to be punished. It's incredibly difficult, if not impossible, to detect and punish intentional feeders quickly without having a decent number of false positives.
That being said, when they do get caught, the punishments are indeed harsher than those punished for toxic chat. They immediately go straight to a 14 day ban, possibly even a permanent ban. They don't get chat restrictions at all, due to the fact that their punishment had nothing to do with what they were saying in chat.