Agreed, if comparing like for like loans.
However I do not agree with your statement...
Actually they do care. A 0% finance scheme is simply a marketing gimmick that disguises the commercial reality of the financing transaction. No finance company lends at 0%. There is an interest cost associated with the finance and that cost is offset against the dealership's gross margin - the borrower is unaware of this as it is undisclosed. In a nutshell the dealership invoices the finance company for the cost of the car, net of the cost of finance.
To illustrate take the example of a car costing €9,000 repayable by 36 monthly instalments of €250 (0% finance to the borrower). Assuming the finance company requires a return of 3% per annum they simply discount the 36 x €250 at 3% p.a. to arrive at the amount they will pay the dealership. In this example the dealership receives €8,597 (say €8,600), so the difference between this amount and the document price of the car is the suppliers contribution to the finance cost i.e. €400. This cost is offset against the dealers margin, which is why they do or should care.
Getting back to the point I was trying to make above re price negotiation; if someone secured a loan at 3% *and* a cash discount of €400, that person would be no worst off than someone taking the 0% finance deal at the full price.