1. Pet Sematary is a wild book! :)
2. As others have said, there are places in the United States where people speak English a little different (with a different accent, too). Places in the Southeast U.S. (Tennessee, West Virginia, Alabama, Mississippi, Georgia) often talk this way and make mistakes. Just look at the title of the graveyard: "Sematary" (spelled wrong :)
Also, many rural people talk this way (in all parts of the United States). Many of these people might not be formally educated and speak "incorrectly" but many of them are very smart about life. They tend to also be good people. Authors will sometimes introduce characters who speak like this to make them immediately likeable to an American reading it. Most of us view people who speak like this as simple but trustworthy.
Many people back in the 1900s (1900-1960) also spoke like this in the United States.
I bet in Spain you can think of low-income places where they speak Spanish a little "wrong" and I bet the people who live there are probably the same way - nice people. :)