PostgreSQL and the SQL standards process

Oracle still does, and it probably won't ever change because it would break far too much software if they did. In Oracle's defense, most languages have extremely poor or non-existent support for nullable character strings, and so this decision tends to make working with Oracle easier in some respects.

In particular, you really don't want to index the "no relationship" values of an optional foreign key. Like an empty email address, for example. To accomplish this in most databases, you have to convert email addresses that are empty strings to nulls when inserting data into the database, and then convert nulls back to empty strings when you are done.

Personally, I wouldn't even like to use a database that distinguished between nulls and empty strings unless there was a convenient facility to do just that.