So breaking all contracts is theft?

Even exploitive ones?

Yes. Theft is breaking an agreement to not take other people's stuff. Otherwise its not an agreement, its a lie.

Ideas are not scarce goods... private property only applies to scarce goods, when copying the owner has his share and I have mine... it may be as you say but it goes against the ANCAP philosophy, in any case it is only my humble opinion

As an ancap, I believe the common ancap philosophy is wrong here. Contracts are central to all transactions. Most contracts are implicit, e.g. If you enter my store, you will not pocket things from the shelf and walk out without paying. I don't think the present system of copyright and patent law is not justified because it is redundant to individual contracts and makes litigation onerous. However the essence of protecting information with agreements is fully in harmony with the fundamental principals of free association and anarcho-capitalism.