The justification for business deductions is that if you don't allow them, you don't have an income tax. The reason is that income does not include necessary expenses needed to earn that income. For a business, those necessary expenses include equipment needed to run the business -- including vehicles. Such equipement is generally deducted over the life of the equipment. This is depreciation.

The tax break aspect is allowing depreciation faster than the rate at which the equipement wears out. Congress has generally allowed depreciation faster than economic depreciation (the rate at which equipment loses market value). The reason is Congress wants to encourage businesses to invest in equipment because that equipment is built and sold by other busnesses. The idea is it's good for the economy overall.

Allowing a one year deduction is obviously quite a benefit. But vehicles in general are depreciable in only 5 years, unless they are used in construction, etc. In that case, the business would have to capitalize the vehicle expense. That means add the cost of the vehicle to the asset being constructed. The vehicle is then depreciated as part of the item being constructed (a building or power plant, for example). Since a Hummer would be a suitable vehicle for companies doing the kinds of things that might require capitalization, I imagine some Hummer sales would not be eligible for this deduction.