which i don't understand.. unless they pose some sort of security risk. however, at that point, it's too late to worry.
At most companies, a lot of employees have potential security risk in terms of access to clients, company files, etc. but a lot of it comes down to spite from a manager or HR. Generally, when a person puts in notice, they go on a mental "vacation" so the two weeks are a wash. While there are always exceptions, this happens a majority of the time so an employer will just cut you to avoid the risk.