I can't understand the rationale of setting the permissions to 750 instead of 640.
I think it's because 640 permissions, in what appears to be a large number of shared hosting environments, may be too restrictive and result in failure.
I think we're both probably saying the same thing, but from different approaches. The issue with the shared server seems to be (as I interpret it) "who can read my wp-config file" - because the database info is stored in wp-config. If you examine the read permissions for 750, then examine the read permissions for 644, you can draw your own conclusions about the "read" bit. Looking at 644 and 750 from a "who has read permissions" perspective only, seems to be quite suggestive of a permissions scheme for the wp-config file on shared hosting.
Something else to consider: Shared Hosting with suexec
"All files should be 644 or 640. Exception: wp-config.php should be 600 to prevent other users on the server from reading it."
Some interesting reading, with some good links;
Who’s Right? Network Solutions Or Matt
More great reading:
shared server permissions wp-config 750 755
If you didn't happen to follow the Godaddy and Network Solutions issues in 2010, you can add "network solutions" or godaddy to that search string for some additional in depth discussions surrounding that, and other shared hosting issues and topics.