Not necessarily. That assumes that the PCs only ever encounter NPCs of their level.
According to the DMG, some encounters (10% I think?) should be up to 4 levels higher than the PCs. Therefore, there's opportunity for higher level NPCs to be encountered with higher level equipment.
By far the biggest challenge in creating opponents is their quantity. If I were to kit out every foe with the full range of PC equipment at higher levels, taking one of the NPCs out would keep a PC set for life! Thus, not every bad guy has the full potential of his equipment, which I think works out just fine.
I also think having one main bad guy (the final villain) be equipped like a PC makes perfect sense. But not every NPC. Certainly not hordes of 5th level pirates, soldiers, and other "fodder" monsters.
To me, this is why it's so important that adventures stick to the guidelines or have good reasons for not doing so.
The other problem is equating equipment value to challenge. So for example, it's very easy to give an NPC equipment equal to his level because he's using or wearing it. It gets trickier when said NPC's equipment value (be it gold, magic items, etc.) is spread out over several rooms. How many bad guys adds up to a more poweful item?
Or to put it another way, if I slay 100 kobolds, does that somehow justify giving an artifact at the end of the adventure? It's a slippery slope.
And it gets slippier when you factor in random encounters, which I really dislike. Random encounters aren't a set number and thus aren't "planned" -- since a monster isn't in its lair, the likelihood of the beast giving treasure is decreased. Therefore, PCs end up with far more risk than reward if the PC doesn't get the treasure immediately tied to that encounter.
Example: if the party randomly encounters a giant anaconda, a shambling mound, and a swarm of spiders (this happened in my story hour), none of which carry equipment on them, should the bad guys days later have more gold/equipment to compensate? How much more?
And in addition, sometimes, especially in seafaring adventures where there's a lot more ability for monsters to wander, the PCs get attacked by a single giant monster (I've enjoyed throwing a giant octopus, giant diving spider, and a sea drake at them). The monster's difficulty is exaggerated -- if you go by the assumption that most encounters are based on four a day, one big encounter in a day means the monster will be more powerful (possibly more powerful than the 4 levels higher rule in the DMG). Should the items the PCs receive later in the adventure be worth more because of that single encounter five days earlier?
Sometimes, I think monsters just shouldn't give out treasure and that's fine. It certainly doesn't justify tossing in an amulet of the planes for their trouble. It strikes me as careless.
In fact, I'd really like some sort of standard where certain "game changing" items show up. I don't particularly find a +5 sword more threatening than +3 sword to my game. More damage is more damage after all. I do have a problem with items that allow PCs to leave the plane at any time being tossed into a reward for a random encounter.
As for calling out which companies/adventures these are, you can find out which adventures they are by reading my story hour...you'll see the PCs acquire these items (and how I deal with it, or don't).
http://www.enworld.org/showthread.php?t=103252
