It's a tax policy designed to stimulate economic activity. (I'm not saying it's good vs bad, just that that seems to be the overall design. If you want to spur investment in new tractors [and employment at John Deere], make tractors eligible for section 179 expensing in the first year, which helps the businesses cash flow [and the government still gets its tax money in later years, when the deduction is $0 for that vehicle instead of a portion as it would be if spread out over multiple years])
