I think we may be confusing issues here. It is correct that the O2 masks with drop at 14,000ft but I think the MMEL limitation is based around certification requirements. The 14,000ft and 15,000ft discrepancy may be linked to the following regulations.
A good reference would be
FAR Part 25 Sec. 25.841 effective as of 07/05/1996
(1) If certification for operation above 25,000 feet is requested, the airplane must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet after any probable failure condition in the pressurization system.
Other possible link
FAR 135 Oxygen Requirements
Occupants: 135.157(a,b)
Pressurized aircraft
If the cabin pressure ever exceeds 10,000 ft msl, the requirements for unpressurized aircraft apply.
To prepare for a possible pressurization loss:
Highest Altitude O2 reserve
> 25,000 ft msl (at any time) 10 min supply for each occupant
> 15,000 ft msl and descent cannot be made to 15,000 ft msl in four minutes - 1 hour supply for each occupant
> 15,000 ft msl but the aircraft is always at an altitude where descent to 15,000 can be made in 4 minutes - 30 min supply for each occupant
An Airbus FAST article
http://www.airbus.com/store/mm_repos...38_Hypoxia.pdf
Hope this helps clear things up.