If you say "I'll give you this scarf if you go to the store for me" and then drape it over my shoulder, I haven't broken any agreement or stolen anything if i walk away. You gave me your scarf without any agreement on my part and will have a hard time enforcing that agreement in court.

Similarly, I suspect that the "by using this site you agree to..." declaration that is widely used has fairly limited legal efficacy precisely due the the lack of explicit agreement.

Even if I do agree to your terms, if I don't go the store it STILL isn't theft. I am merely in breach of contract and you will have to go to court to get them to force me to return the scarf you gave to me.