For ordinary users, deleting everything immediately when someone tampers with it is a recipe for disaster. Sure, they can backup everything in encrypted form, but then the data is not really deleted when somebody tampers with the machine, isn't it?
Regarding the security, well, apart from software-based attacks, how about installing a tiny USB keylogger inside a USB cable that is already used by the user? Or in the keyboard itself? Or a camera that records your keystrokes?
That's what the <insert agency or special interest group of your choice> would be doing in such a case.
Not all products are the same. It's irresponsible, but maybe good business strategy, to presume a complicated computer, which runs software that no human can hold in their head is the same as other products people are accustomed to.