I think you are taking a simplistic view of finance. From the standpoint of a company issuing the loan, there are (at least) two "costs" associated with loaning money. (1) The risk of someone defaulting, and (2) the opportunity cost. That is, how much you could earn with your money if you invested/used it in a different manner.
Neither of those costs scale with volume. The same percentage of people will default, regardless of the number of loans, and the percent return on your money if invested elsewhere will largely be the same regardless of the amount you decide to invest.
Therefore, the "cost" the company has to absorb to finance your tractor has almost nothing to do with the number of units they sell. It doesn't get cheaper for them if they sell more units.
Any company that offers financing has to recoup those costs, or they are literally losing money on the transaction, and they won't be in business long. If they just break even, it still isn't a good investment, so you can be sure they are going to charge a little extra. So, yes, there is no such thing as a free lunch (0% interest). Someone has to pay for those costs, and it's most certainly the buyer.
If you don't believe me, go into your bank tomorrow and ask what the interest would be if you want to take out a 30 year, $200k mortage loan. Then ask if you can get a 0% loan for the same amount over 60 years instead, or if you can get a lower interest rate if you borrow $400k instead. Be prepared to be laughed at.