Why should the DM have to justify reason when the players are trying to abuse the rules?
Because they aren't abusing the rules, merely using them. If you know a spell that grants everyone +1AC until hit, it's a fine idea to use that spell
before combat. In fact, armors with such properties actually exist. If a leader has an ability to prevent damage via temporary hit points rather than healing, he's not abusing the rules by doing so beforehand. A DM builds a world and with it a framework for what is reasonable in that work. If day-long buffs are reasonable, then that's not abuse, just due diligence. I mean, the vast majority of 3.5 casters essentially did this kind of thing every day - a bit of buffing isn't abuse
per se.
The point is, the DM controls the framework and he can (and should) change it when it's not doing what he wants. If you don't want players buffing up hours in advance, don't tell them not to; just don't let buffs like that last for hours.
Nope. You simply say, "The calisthenics have reinvigorated you, (reconditioning your muscles, focusing your mind, opening up you soul to inspiration, depending on power source), counting as a short-rest. Would you like to spend any healing surges?"
Certain levels of meta-gaming are silly. In general, having temp HP going into a fight doesn't bother me, but they have to make the sacrifices involved for that to happen, not try and game the system.
First off: the difference between your solution and mine is mostly one of flavor. Both of us seem to agree that you should not be able to save temporary hitpoints by avoiding short rests.
But I still believe that there's a big difference in implementation. What
is gaming the system anyhow? If someone's dazed and you shift a square away so he can't charge you nor hit you normally, are you gaming the system or just playing tactically? If you assume that a fire breathing reptile is likely to be resistant to fire and perhaps vulnerable to cold, is that gaming the system or playing it safe?
The artificer keterys first mentioned, also has an ability to buff magic items long before combat starts; it's clearly intended to be used as a kind of "buff before fight" effect. Why is that buff good and the temporary hitpoint buff bad? Basically, the artificer can grant someone extra power at the cost that this power only lasts until the target needs to "refocus his mind" during a short rest. The artificer can himself rest, and given enough time and surges, he could buff everyone except himself before combat - is that buff "gaming the system"?
I realize that no perfect solution exists. But these questions will arise time and time again for THP granting powers used outside of combat. Discerning valid buffs from "gaming the system" risks being arbitrary and capricious. There aren't any mechanics or guidelines concerning forced short rests as far as I know, and it looks like there's the assumption that the ability to short rest is a boon, not a bane.
I believe we agree that many such buffs are not intended to be effective long before combat. The choice that each DM must then make is whether it's better to force short rests, rule case by case, or limit the duration of THP. I believe the last solution is best both for gameplay (everybody understands it and knows in advance what the DM will rule, and there's no discussion about DM's controlling PC's), and for consistency (no need for case by case judgments which are likely to be at least somewhat inconsistent). It isn't perfect - for instance invigorating assault still is a bit problematic, and it may remove a few otherwise harmless options.
I don't expect everyone to do it this way, but I hope I've clarified that I'm not trying to encourage abusive behavior, I'm just trying to make a clear and fair guideline that can consistently be applied and avoids needing too many rulings in that inevitable gray area between use and abuse.