In th book The Giver, why has Asher beeen punished for confusing the words snack and smack?

In the Community of that book, precise word usage is very important.  We see this in the beginning, when Jonas struggles to figure out how he feels about the upcoming ceremony, finally settling on "apprehensive."  Children are taught from an early age to only say things they truly mean.  Asher says he wants a smack, so they smack him; this shows him that he should be careful what he asks for, and to be correct in his pronunciations and word usages. 

Though this example seems a little extreme, especially for a toddler just learning to talk, it illustrates a philosophy of their community.  In another scene, we learn that Jonas was scolded for using the word "starving," when he really just meant "hungry."  The community does not want its citizens to ever think they could starve; they are self-sufficient and so thoroughly organized that starvation has become an impossibility.  But to be reminded of this, children are taught not to use that word.

Later, when Jonas becomes the Receiver, he learns that most of the community misuses words, especially emotional words.  He understands that only he and the Giver have ever felt any real, strong emotions; everyone else has only felt a faint glimmer of that.  Their lives are so sheltered, so orderly, that they don't ever experience anything extreme enough for a true range of emotions.  So really, even though the community strictly enforces correct word usage in some ways (the smack vs. snack issue), in other ways they gloss over it (emotions).