In my opinion, when something begins to take over our lives and takes priority over our relationship with others and more so with Christ, then it's an idol.
I like my nativity scenes. They're pretty on the china cabinet and my kids love to play with the Fisher Price one. They're not taking over our lives. They're not taking a huge place in our hearts. We do not bow down to them nor do they take time away from the Lord.
They are not idols.
As others have said, when you take those verses to say that nativities are evil, then ALL forms of images and likenesses are evil. Not just crosses but photos, dolls, stuffed animals, etc. My daughter's artwork would be evil and sinful. My little one's doll house dolls would be sinful. IF they are idols to you, then don't play with them. But you cannot say that they are evil for everyone else. Period. Hey - even GOD said to make a likeness of cherebum on the Ark - so I think there ARE distinctions to the second commandment.