Primarily because work computers contain gobs of intellectual property. That's to say nothing of all the sensitive security, passwords, and customer/client data that exists on corporate/company laptops. Whereas school computers (at least up through high school) do not have any of these risks.
The difference here is that corporations lock down computers to protect the corporate IP and sensitive data, whereas this article is talking about locking down computers to prevent it's user from using it immorally. This is a problem because the school can't implement moral restrictions with which all parents can agree, and that could become a legal quagmire.