because those bitcoin must be digitally signed to you for you to have possession. Bitcoin possession = ownership

In addition to the elements of a contract:

• a party must have capacity to contract;
• the purpose of the contract must be lawful;
• the form of the contract must be legal;
• the parties must intend to create a legal relationship; and
• the parties must consent.

If a contract is in a written form, and somebody signs it, then the signer is typically bound by its terms regardless of whether he has actually read it,[28] provided the document is contractual in nature.[29] However, affirmative defenses such as duress or unconscionability may enable the signer to avoid his purported obligation. Furthermore, if a party wishes to use a document as the basis of a contract, reasonable notice of its terms must be given to the other party prior to their entry into the contract.[30]