Yes, this is often overlooked, and one of the main downsides to NoSQL databases. Schemas don't really go away they just get pushed into the application layer. When multiple apps use the same database, schema maintenance and evolution become a real issue.

Perhaps it's because much of web scale data is arguably tissue paper data, more useful in the aggregate. Transactions and constraints on the data seem to be less a concern.

In this regard I think key-value stores are preferred over document stores such as MongoDB and CouchDB. A key value store makes it clear where the semantics of the data belong.