I think the whole 100% off parts thing was part of Turn 10's plan to discourage money glitching. Me and my cousin were looking at GT5 money glitches on youtube and then on the side it had all sorts of Forza 2 and 3 money glitches; all of which I was unaware.
I did know the FM1 glitch were you had to convert the engine of a certain car and then sell the engine back or something. SO I think this is why they give you 100% off parts so easily, because all of the FM glitches we saw had to do with buying and selling
^^^ That is why I doubled the price of my tunes from 5K to 10K. When you don't have to pay for credits, there's really not much left to do with your credits, apart from spend it at the storefront, and buying cars.