If you're a PC, you count as a cleric if you have cleric levels. But monsters don't have class levels, not even humanoid monsters. An Orc Eye of Gruumsh is not a cleric, it's a monster. But since it has the Spellcasting trait and casts spells from the cleric list, it can attune to magic items as if it were a cleric.I wrote what the DMG stated, so they used "monster", instead of "opponent" or "creature" or "foe".
So you're saying that monsters should be the main focus with attuning to magic items, so the DMG went with focusing on them over the PCs? They could have chosen the word: foe, opponent, or creature in its place but used monsters.
Similarly, an Eldritch Knight fighter is not a wizard, so they can't use a staff of frost. They cast (a subset of) wizard spells, but that doesn't make them a wizard. They are, however, spellcasters, so they can use a wand of fireballs.
If the designers had intended that a wand of fireballs could only be used by sorcerers, wizards, and fiend-pact warlocks, they would have said so in the attunement requirement, much like on the staff of frost.
Rangers are already spellcasters (from 2nd level on), so there's pretty much no interpretation of the rules in which they can't attune to an item requiring a "spellcaster."Aramis Erak made a valid point, what if someone took the Magic Initiate Feat, let's say a Ranger, then he could use a Wand of Fireballs. If you think that's fine to do in your world, go for it. I'm not going to do that in mine.