Why wouldn't the average user? The entire idea is that you'll just have to remember two passwords: your computer account, and your password manager. At least for most users, the idea that some password shouldn't be stored just opens the door to bad practices and password reuse.

For someone working on a password manager, I think the default assumption has to be that a screwup on your part will--literally--impact pretty much every aspect of a user's life. You can't assume that some passwords won't be stored.