Auditing software is hard! The most-heavily scrutinized smart contract in history had a small bug that nobody noticed — that is, until someone did notice it, and used it to steal fifty million dollars
Blockchain is not only crappy technology but a bad vision for the future
Kai Stinchcombe

Auditing code is likely easier than auditing legalese. 
In other words the analogy that buying a book through a smart contract involves the smart contract review is equivalent to that of having to review the Amazon terms of use when ordering through Amazon. 
Once a (legalese) contract is standardized everyone can deal with its abstract meaning.