Why are fighter jets rated differently than passenger jets?
The philosophy of fighter cabin pressurization is different from that of the passenger aircraft. In case of airliners, the cabin pressure decreases while the aircraft climbs upto a certain altitude, after which the value is kept constant (usually at 6000 - 8000 ft cabin altitude). 

In case of combat aircraft, the principle is different. The cockpit pressure is maintained at certain value to a particular altitude (usually 8k feet); then the pressure is maintained (at 8k ft) till the aircraft reaches another altitude (~23k feet); after that a pressure differential (~5 psi) is maintained between the cockpit pressure and the ambient pressure. This reduces stress on the airframe and is safer in case of rapid decompression during combat. So, a fighter cockpit at 40k feet is actually stressed less (due to pressurization) compared to an airliner at that altitude. 

There is another thing to be noted- a combat aircraft is designed for much higher loads compared to a civilian airliner; so, the structure is capable of taking more stresses. So, pressurization is not the limiting factor in case of combat aircraft.



One way of calculating the life of fighter aircraft is to apply loads on the airframe depending on the expected mission profile (like TO- cruise- loiter- cruise- landing), then calculate the theoretical life based on how long the airframe survived. Of course, this means that the life will change if the mission changes- like what happened when some aircraft were shifted from high level bombing to low level penetration roles.