It's all in a person's viewpoint really. Someone who invokes, or wants to invoke, dark entities such as demons, may not label them as 'evil'. It's the same concept as 'selling you soul to the devil' and things such as that nature I'd think, where the person who is doing it is doing it to fulfill a desire, be it power, sex or otherwise, and is not looking at the moral standpoint of good and evil. It's a way to get what they want, that's all they care about.
Also, a lot of the spells I've seen are ones that won't work. Since anyone can write spells on the site, what gets posted as a spell or ritual is not filtered well.