It's a combination of intimidation, to force you to buy overpriced, unecessary insurance, which they almost never have to pay out on (for one thing, between premium credit card perks and your own policy, the coverage is usually there, anyway), and stealth price increases. By telling you that you cannot drop cars off hours without complications, and in the same breath, letting you know how painless it is to extend the rental, they are figuring you will opt for the latter, to avoid problems, and push yourself into another day. It's the automotive version of long distance companies who round up to the next minute, even if you are only a second or two into it. Screw them.