It's OK if you and others don't think it's fair. We just disagree on that aspect. I don't think it's fair for an employee to use that particular method. To me, it comes off as a kind of extortion to the current employer, and mistreats the employer who made the offer.
Others can, of course, have a different opinion. Nothing wrong with that. We simply disagree.
To make a little meta-comment: I am genuinely surprised at the negative reaction I've gotten from a few people here. I really didn't think I was saying anything all that controversial. Live and learn, I guess!