Not entirely correct. OBD only mandates emissions information to be made available in a standardized way.

There are plenty of proprietary codes that might set a malfunction light and not show up on an OBD reader, or not be interpreted by it.

(there are tools that reverse-engineer the proprietary protocols that can show those codes, but they aren't $20 - more like $200 and up)

I really don't see why you're defending hiding information. Even for someone who doesn't want to mess around and would just take it to a dealer, making the information available without the need for a code reader will not hurt in any way.