Because he's a strict Constitutionalist. I would suggest a review of his actual written opinions. They are spectacularly in favor of a strict interpretation of the Constitution. His Obamacare dissent is a great recent example: the civil liberty to not be compelled to buy a product.

Also look at Texas v Johnson, Maryland v. King, Kyllo v. United, Florida v Jardines.

Scalia would almost certainly be on Apple's side in this because it represents not a question of legislation but a question of the power of the state.

It's easy to dislike Scalia based on decisions not going the way one's personal convictions would hope, but in questions regarding the power of the state, Scalia is very clear. Remember Scalia defended flag burning as well as opposing thermal imaging into a private home without a warrant. He also opposed involuntary DNA submission -- definitely read the opinions and not simply examine the outcomes.