Fridge Brilliance In the remake, The Devil shows up after Elliot lusts for someone else's woman. A violation of one of the Ten Commandments.
If I recall correctly, Elliot mentions at the beginning of the film that Alison's recently broken up with her boyfriend. She's back together with her boyfriend at the end because of Elliot's own wish that she have a happy life.
Arguable. Elliot points out at the start of the film that Alison's romance has been "over for months now", so it's not unreasonable to think she's moved on. Furthermore, it seems more likely the situation was Fridge Brilliance of a different kind; namely that the devil, as a being of pure evil, is incapable of doing good. Thus, if someone makes a truly virtuous, selfless act, the devil would be unable to grant it; so, instead, she wrote an escape clause in the contract, voiding the whole thing if anyone tried to make her do so. It's less likely that the devil actually gave Allison a happy life and more that she simply refused (or was incapable of granting) Elliot's wish.
Alternatively, Alison is gay. She never actually says that she's dating a man, after all, just "seeing someone". Plus, if we go with the Old Testament idea of homosexuality being a sin, then if the Devil gave her the courage to accept that part of herself, than it would fall within the Devils ability to tempt towards sin, since Alison would be committing sins more often. Actually, that might be Fridge Horror...
Fridge Horror: In the remake, the escape clause has made me wonder. If it requires a selfless wish, then no one who has read and understood the contract could escape it. Think on it, a person who knows they need to be selfless will be looking for a way to help themselves in the long run. They are out to save themselves and that is not selfless. Thus anyone who tries to understand the full extent of the deal ends up losing the only escape clause.
It could probably work if the selfless act was spontaneous.
Fridge Logic: In his review, Roger Ebert questions why the devil goes out of her way to be a Jackass Genie, causing Elliot to want out of the deal. Wouldn't it be better for her to genuinely make his life wonderful, so he will recommend her services to others? Arguably applies to the original film as well, but there the devil had a concrete goal and doesn't need to keep drumming up new business in perpetuity.
Evil Is Petty probably. Also the ending suggests she's not that bothered with losing the soul in the end.
She's trying to make him use up the wishes as quickly as possible, so that she can get his soul.
This being the devil and all, she simply may be incapable of granting truly virtuous wishes that improve people's lives. Hence why she wrote an escape clause into the contract that triggers when someone makes a wish so pure-hearted, no amount of Jerkass Genie-ing on her part could screw it up.