Like I said, the burden is in a form of a social more, not any concrete law. If employees are expected to give their notice, and put up with their employer in the same way as they have for the normal duration of their employment, then that is a burden since the employer does not have the same social expectation when they are letting the employee go.
The employer can say You wouldn't walk out on us now, would you? That would be a dishonourable thing to do (probably in a less direct way). An employee doesn't seem to have that same kind of social more leverage.
EDIT: This includes the circumstance that you mentioned - the employer is being somewhat unreasonable - since they can still guilt the employee into pushing through it. Another thing to consider is that, while the employer might not be treating the employee worse than he used to, the fact that the employee feels that he has to stay for the extra two weeks may be an inconvenience to him. He may have things that he want to take care of, etc.
The employer can say You wouldn't walk out on us now, would you? That would be a dishonourable thing to do (probably in a less direct way). An employee doesn't seem to have that same kind of social more leverage.
EDIT: This includes the circumstance that you mentioned - the employer is being somewhat unreasonable - since they can still guilt the employee into pushing through it. Another thing to consider is that, while the employer might not be treating the employee worse than he used to, the fact that the employee feels that he has to stay for the extra two weeks may be an inconvenience to him. He may have things that he want to take care of, etc.