I'm not sure why anybody thinks this is a bad thing.
It is NOT that Apple wants you to buy their overpriced RAM, and it isn't that Apple doesn't think you need any more. That's ridiculous.
As philipma1957 said, Apple isn't in the RAM business and doesn't want to be. They can't afford to sell you fast RAM at good prices, so they offer a low minimum so you can buy it from somebody who can provide it at a reasonable price.
Apple knows the majority of people who buy Mac Pros know how to install RAM and know where to buy it cheap. With the Mac Mini and iMac they can't make this assumption, hence the higher minimum despite the generally lower RAM usage on these machines.
If you want 32GB to come installed in your order, Apple will let you, but they aren't interested in being criticized for overcharging for something that the market isn't interested in paying for at all.
The same thing goes for hard drives, although it's less obvious. I'm actually under the impression that Apple's SSD prices are reasonable, even if there are faster ones available from other vendors.