This has been discussed about 20 times but there are vents under the front seats for the back. There is a photo in one of the original press releases and I can verify there are definitely vents aimed towards the back directly under the seats.
1995 325i,1996 328ci,1997 528i,1997 Z3 2.8,
2001 X5 3.0,2001 330i Conv,2002 M3 Conv,
2003 M3 Coupe,2004 M3 Coupe,2004 Z4 3.0,
2004 X3 3.0,2007 X3 3.0,2007 335i Conv.,
2013 X1 28 sdrive M Sport