IMO, it's less an issue of lore and more one of game mechanics. Much like the recent confusion regarding whether or not player character Death Knights are alive or undead, folks have a tendency to twist themselves into knots trying to explain quirks of the game mechanics within lore. With the DK example, it's exactly like the Forsaken--they're really most sincerely dead (it says so right in the DK intro), but are not classified as such in the game, due to the imbalances it would create in the game.

I'd imagine the same is true WRT druid spells being classed as arcane, when lorewise Night Elves and Tauren are so adverse to arcane magic. It's probably some sort of weird game balance thing at play.

If you read the Druid spells names you'll that they're assosiated to heaven and night, and, thus, to Elune - as I understand, since she's a night Goddness of the Night Elves.
The similar question was asked earlier in WoW Insider, as I remeber. And it's said there that there's no reason to invent a new magic scholl for 2 druid spells (logically ideal answer). I prefer to think that those spells are a kind of plea to nature and Elune, rather than simple mastering of arcane the mages use. The animation of the spells also shows them as heaven-sent, which connects them to Elune