My reasoning is because the distinction is arbitrary enough as it is. Particularly when it comes to playable monsters, where different rules land you with a PC who is generally a "lesser" version of whatever their race is, but better at general adventurerness. But, mostly, it's about elegance for me. Having different rulesets like that smacks of a kludge, like making non-combat NPCs and objects immune to damage. Ideally, the system should have support for letting players take over any creature in the game, provided it's appropriate for the power level - even mindless things could be one-off temporary characters or vehicles for the players who only want to kick in doors and murder up some monsters.
That said, there are settings where giving the PCs special rules is appropriate, usually when the PCs are supposed to be special people. In a campaign I ran a while ago, the PCs were gestalt. Most NPCs weren't, because the PCs were the Big Damn Heroes of the entire setting, and wound up becoming living gods by the end of it. Similarly, and on a lesser scale, the max HP thing. They don't get max HP just because they're PCs, but because they're badasses who're much tougher than anybody has a right to be. Put another way, they're PCs because they're the jackasses lucky enough to have rolled max on all their hit dice.
Selection bias can account for differences like that, but from what I've heard, in 4E the systems are completely divorced - for monsters, you assign stats as you need to and toss in one or two each of at-will and encounter abilities, and then cross-reference with the guidelines for what a monster of a given CR ought to have for its numbers; all of this is basically arbitrary "Okay, let's use whatever works" design. Put another way, it seems like 4E basically doesn't have a system, and relies on Rule 0 to fix that. For players, you follow the system of character creation. That said, I've not looked at 4E since it's release, so there are likely dozens of people who are more familiar with it and with less hazy memories that can correct me.
