Yes it is. If I lend you $10 and you refuse to pay it back, it is generally deemed "morally okay" for me to be mean about it. Courts, and by extension, police enforcement of court rulings are that principle writ large.
The whole concept of ownership is based on social contract. What is "mine" versus "yours" arises from that contract. Theft, because it is rooted in ownership, is thus a relative construct.