Question: I know everybody tells the truth, but why does everyone say what they are thinking? If you don't say what you are thinking it doesn't mean you are lying.
Answer: It's an imaginary world with its own logic. You could theorize that people were raised this way. Also, you could imagine that since they have to say bad things to answer most questions, why not say bad things all the time, i.e. if your wife asks if you like her dress, you have to say you hate it, so why wait to be asked. Really, the film makers did it because it's funny. If you made a movie where people only spoke when they had something nice to say, it would be very boring.