It's not against their will. It's a part of the contract they signed when they started working at the company. The contract stipulates how it can be terminated (in accordance with the local law). If it says each party can terminate a contract with a prior notice of two weeks, the contract is enforced for those two weeks after giving in the notice. There's still an employee-employer relationship at this point, even if the employee gave their notice.

In some countries the notice period can go for months. Usually it gets longer with the tenure. It allows both parties to transition and prepare in advance.