So, I ask again: Is it fair to set an NPC's hp to maximum hp? Is it fair to do that *after* initiative? Is that *really* that different? Is it fair to set an NPC's hp to higher than maximum hp? Is it fair to do that *before* initiative? So, is it fair to set *all* NPCs to maximum hp?
Yes. No. Yes. Potentially. Potentially. I don't understand the question. (In that order.)
Unless otherwise stated, "normal status is full health" is the default assumption of every group I've played in, witnessed, overheard, heard about, or in any other way received any information regarding, ever. So when an NPC is made to actually "exist" as a mechanical implementation rather than purely a narrative character (using my previous meaning of "exist"), that NPC will be assumed to be in default status. If, and only if, there is a narrative justification for deviation from that status, then there will be a change. For example, it's our second fight with a particular enemy in as many days, but this time the enemy has been given a dangerous alchemical potion that temporarily lets them shrug off wounds (perhaps with nasty, but delayed, side-effects). This is a perfectly valid narrative justification for a being which "exists," mechanically, to deviate from its default normal state (full health) to a different state (beyond normal health). Alternatively, we kicked this enemy's butt pretty hard last time, and for whatever reason they didn't get a chance to recover. That would be a perfectly valid justification for going the other way.
These changes CAN occur after initiative is rolled, but I consider it relatively unlikely that such narrative justifications could simply appear in the course of combat. Not impossible, but unlikely.
It really, truly is that different. Again: One is the DM deciding what will
enter into mechanically-realized "existence." The other is tampering with things that *already* exist. Such tampering, unless it is explicitly called out (via narrative justification, presumably) inherently invalidates the players' ability to make informed decisions: the world is not what they think it is, but they are intentionally denied the ability to
learn that the world is other than what they think it is. If the difference is in their favor, they will not learn what choices are bad or dangerous; if it is against them, then even their good choices may be thwarted. Either way, they will have a mistaken understanding of what makes their choices good or bad. And yes, I still hold that even knowing that "good choices" can be invalidated by especially unlucky rolls (from the players' perspective). If the choice involves a percentage chance of risk, that percentage chance
should be factored into the choice; if it was not, then a lesson can be learned about accounting for the chance of terrible things maybe happening (which, over the long run of many D&D sessions, is essentially guaranteed
eventually).
Edit:
Also, to explain the "I don't understand the question," I think I do know what you meant, but I see a different and equally valid interpretation of the question, and which interpretation I take would radically change my answer, so I cannot answer the question as stated. However, I hope that my above explanations have given an adequate answer nonetheless.