The person making the promise, promises to do something or agrees to something if the action he wants done is completed by another. In the advertisement for reward context, by offering the reward, someone if offering a unilateral contract. In the UK approach, the person promises to pay a reward, should anyone fulfil the obligation stated for the reward. The only person who is specifically responsible is the person who promised the reward. No one else could be held liable for inaction, only the promiser could be held responsible for inaction.
In the German approach, in an advertisement promise, when another person fulfils what has been advertised and the promiser refuses to fulfil his promise, then there is a breach of promise. The breach of contract and promise are both considered in their respective courts Both are enforceable in court. For example, a unilateral contract is enforceable when someone chooses to begin fulfilling the act demanded by the promisor (the UK legislation in the case of advertisement for reward). A bilateral contract is enforceable from the get-go; both parties are bound the promise.
A unilateral promise standpoint as in the case of the German legislation expects that the promise made for a reward holds the promisor legally. The Chinese legal standpoint in terms of property would consider the advertisement for reward as a unilateral promise. However, in the case of non-property related promises might be willing to analyze the contractual nature of the promiser and the promisee.