The problem may be that regardless of what is in the contract, an employee has effectively no rights in the first 12 months of employment, and can be dismissed for no reason. Even after the 12 months, it is not that hard for an employer to come up with a reason to fire the employee or make him redundant.
The same works the other way round too. If an employee decides to bugger off before he works his notice period, there is very little that an employer can do about it.