The government deals with gazillions of pages of documents and it is inevitable people are going to go "oops" occasionally; a page slips out of the stack and lays in the bottom of a hanging file drawer for a couple months. Usually this happens with the less-critical documents that NARA isn't so worried about. If you purposely wheel an entire file cabinet full of nuclear secrets out, that's likely to not be an accident.
I have made the same point with library books. Library books are not your property but you can borrow them. It might slip your mind to return it but if you keep them past the due date you may incur a small fine upon return. If you keep them for years and don't notify the library, then they start to consider that you've stolen their property which is a big deal.
I have told the story about forgetting to return a music score to my state University library over the summer and bringing it back two months later with apologies. They were quite happy because apparently not returning a book to a state-run library is "stealing from the government" which is a felony which would have been a lot of paperwork for them. Because I didn't go "oh well, they've probably forgotten about it" I just paid like nine dollars in overdue fines instead of getting arrested the next time I ran a stop sign.
Same thing when I interviewed to work at the IRS they talked about how they know people are behind and have problems with back taxes, but if you stay in communication with the IRS and let them know regularly what's going on it's much less of an issue and they'll delay going through the legal process of garnishing your wages. If you don't communicate, they can only assume you're on the run and have to take the drastic measures.
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