It's occurred to me that they pulled the switch on us in the 3rd act when they realized they totally shot themselves in their foot with unlimited, craftable wailing walls. I'm sure not going to complain, but I know I jumped up from like a 55 or 60% bonus to a 175% bonus. I'm never going to have to use my donuts on something just for the percentage bonus again (which is nice, now I can buy certain items just because I like their looks instead of going 'yeah, I like the design, but for that much I can buy X amount of X and get a larger bonus').
Point being, I think it might have been a way to curb our zombie/brain hording.