I absolutely do understand that. But there are economic principles at play here too. A $7,500 rebate is effectively a lower price without costing the manufacturer anything.
While it is true that the buy has to meet the $7,500 threshold for that to work (which is not insignificant) for much of the lifespan of rebate availability, the cars were averaging >60k. Hopefully most people buying cars in that price range have incomes that make $7,500 in tax rebates possible.
By the time that the prices had dropped to 50k, the rebates where closer to $3,700. How many were sold at each range and what percentage of the rebate was accessible? Hard to say... thus the range of possible subsidies.
Either way, though, this was effectively a big help to the business.
Second, the buyer has to owe enough in liabilities to be even be eligible for it.