No fear of arrest. No fear of prosecution. The worst-kept secret on the border for years was that, until more recently, U.S. authorities would not even try to prosecute cases involving less than 500 pounds of marijuana.
Furthermore, U.S. laws and rules of evidence that aim to ensure justice make some cases hard to stick. The most trouble that many small-time smugglers faced was a few days' inconvenience before being deported.