There's basically zero reasons for night elves to enjoy the presence of demons. Demons have screwed night elves over a few too many times, managed to get the world blown open, and had their way with Nordrassil one too many times. Still, other races find uses for demons. Humans summon them like idiots and blood elves made heavy use of demonic magic. So what's the night elven attitude on demonic minions?
Well, they seem to tolerate them.
Edune family cottage. There's four night elves living there along with two humans. One of the humans is a male who appears to be a warrior. The other is a female, a human warlock. How can you tell she's a warlock? Well... Her imp. Her imp is sitting right by her as she sits at the outdoor table.
So what's the deal? Why would night elves, who have put up with so much corruption and shit from demons over the years, deal with this imp just sitting so close to their house? Perhaps they recognize the imp is under someone else's control and that it should be okay. Or maybe it's a property issue; the imp is, after all, not theirs to command, so should they bother the warlock to dismiss it? Perhaps the night elves simply prefer to keep it in sight so they know it isn't off causing trouble. I really can't tell. However, Aleanna Edune doesn't seem too taken with the warlock:
"Hmm... well, just be careful, he might be sweet on you. I don't trust that sister of his either..."
Obviously, there's some sort of difference. After all, a demon of the Burning Legion and a warlock's imp minion are two very different situations to deal with, right? Or are these just more tolerant night elves than those in, say, Darnassus, where a warlock trainer is nowhere to be found? (Yet they have a portal to the Dark Portal there in the Temple of the Moon) Then again, Darnassus sits on the same tree that Zenn Foulhoof, a satyr, is allowed to just hang out by the road... Out in the woods. Y'know, near all those awesome sacred trees.
I'm sure it's just all a result of those bad influences the night elves' human allies are putting on their people.