That's just the way it is, and the way it always has been, all across the industry. Consumer models ship with the minimum RAM. It wasn't until '99 or '00 that all of Apple's machines started shipping with more than 32MB, and that was about 4 years after 32MB was below the "practical" minimum.
Today, the practical minimum is 512MB. But it's usable with 256, so that's what they ship. More standard ram = higher prices = fewer sales = bad. People who need more can always add more, so it doesn't make sense to include more than you need to as standard.
Hey, at least they're not as bad as Dell. Most of Dell's low-end machines only ship with 256MB, and 64MB of that is used as video memory! Ugh.
Apple's started moving to 512, though. I'll bet all their systems will ship with 512 standard by the end of next year....when 1GB will probably become the practical minimum.