FAA Advisory Circular (AC) 25-13 (dated 1988) is frequently used as a means (not a "rule) to certificate aircraft for reduced thrust takeoff. You'll probably find it on a dusty e-shelf at their website (
www.faa.gov, or something). There is a JAA equivalent, which is very similar.
Section 5:
....you can use flex if......
Paragraph 5e:
"A periodic takeoff demonstration is conducted using the airplane's (full) takeoff thrust setting and the event is logged in the airplane's permanent records. An approved engine maintenance procedure or an approved engine condition monitoring program may be used to extend the time interval between (full) takeoff demonstrations."
So, there you have it. It is likely that the use of flex on any given type is certified on the basis that a full thrust takeoff is done every now and then. However, somebody may have proven that the means to extend the period are so robust and trustworthy that it's been stretched to infinity (and beyond?).