I think talking about the Leadership Principles as a 'typical corporate motto' is unfair. Amazon lives their leadership principles far more than the other companies I've seen. They don't include things they don't actually care about, like most corporate mottos do (you'll notice the lack of 'we care about our employees' or 'do no evil' LPs).
At least in my experience, you only need to pay back Amazon proportional to (2 years minus duration employed) so it really isn't that bad (maybe that wasn't the case when this was written).
Maybe it's changed, but the signing bonuses I saw were essentially increased salary until your stock starts vesting seriously. It gets paid out and vests every paycheck so there isn't anything you need to pay back.