Why do egg white cartons warn that pasteurized egg whites shouldn't be used for whipping or meringue?

Egg white foams are a delicate thing. They are easy to do, but also easy to mess up, and many things will do so. Here (Wayback Machine) is a blog that addresses many of the common or less so do's and don'ts of foams and tests them to see if the writer agrees with them effecting things. Items addressed are should eggs be new or old, at room temp, when to add sugar, and if pasteurized eggs work. For them, pasteurized was fine.

The general idea behind not using pasteurized as I understand it: For quality foams, the proteins in the egg white must unfold correctly during beating. In the process of heating the egg for pasteurization, the protein may well be chemically or physically altered and the ability to unfold it correctly for a foam might be reduced. In their test, it worked fine, just as it seems to have for you. But, at best, it may not be as consistent or forgiving. If you are slightly off on another aspect, say whipping speed, it may increase the odds of failure. Also, you may have gotten a batch of eggs that were pasteurized at an optimum temp, while another batch might have been done at a few degrees higher and affected the protein strands more and that batch might fail. I personally would consider it one more factor that might increase failure rates, might increase the need to add cream of tartar or lemon, might fail with sugar, etc.

Just opinion, but if it works for you, go for it, but expect that maybe some times it won't.

While pasteurization could denature proteins if the temp goes too high, the eggs whites generally shouldn't be getting hot enough for that to really happen.´

This assumption is incorrect. Denaturation of eggs is not as simple as, say, a phase transition of a substance. Each protein species in an egg has multiple possible states on a spectrum of unravelling. Temperature is a statistical measurement, which gives you the average energy of the molecules in the egg - but each molecule will have a different amount of energy. Also, the same amount of energy in the same molecule might be able to lead to two different states, depending on which hydrogen connection broke down first. Add to that the interaction of those proteins with all the other stuff contained in the egg, and the fact that egg white has dozens of different protein species, and it becomes clear that there is no single tempperature at which all molecules in the egg white suddenly change their state.

Instead, you have a wide range of states, from almost-like-raw to cat-played-in-a-box-of-string-ends (or maybe velcro pieces). And pasteurization certainly doesn't give you cooked-through-eggs, but it does cause enough changes that the egg's behavior is no longer the same as with raw egg whites.

Age is not the reason behind the advice. Aged egg whites beat up nicer, as the proteins are somewhat pre-relaxed there. In fact, the most finicky egg white foam applications (like macarons) might include an aging step for the egg whites.