One justification for companies to do this, is to ensure the employee doesn't damage the company property or reputation. An employee told 2 weeks in advance that he will be sacked is a dangerous mind.

What i never understand though, is companies forcing employees to train their replacements, after which they are fired.

Perhaps companies ought to publicize their employees records or performance, if only limited to qualitative ones, so that during quit time, employees are not blackmailed for getting a good record of employment.