It's the IRS' official position, which they are communicating to tax software companies. I know someone who knows someone who works for a non-Intuit tax software company, and that person reports that the IRS is telling them the same thing. "[My] contact at the IRS said that they won’t accept extensions after 5/17 so if you cannot file by 6/15, you have to file your EXT by 5/17.  The End."

Why they can't clearly communicate that to the general public, I do not know.