The rules of employment contracts are similar to the rules of contract in general. For example, as per the freedom of contract, one is fully entitled to choose whether or not they want to enter into a contract. This applies to employment contracts as well. Employers are free to decide whether or not they want to enter into a contract with you i.e. hire you. Just like in regular contracts, the principles of offer, acceptance, consensus ad idem, etc are necessary to form an employment contract. An employer cannot unilaterally impose a contractual term onto an employee.