Because the US still imports 5 million barrels/day of oil, and consumers spend a substantial share of income on energy, cheaper oil boosts GDP. Every $10/bbl decline shaves 25 cents off the price/gallon of gas and reduces oil imports by $50 billion/year. That will boost GDP by almost half a percent. If oil falls below $65/barrel, the benefits remain positive but shrink as US drilling and exploration activity will fall.