There's a big difference between the two. In the book case, they have a fan in an entirely different business who infringed. They're sane enough to realize that this is very unlikely to impact their bottom line, and there's no malicious intent.
In the "Jack Dawson" case, it's naked profiteering off the original brand's name recognition, and it's clearly malicious. Why be nice?
Assuming it ever came to court, it comes down to who the jury likes. Compare this letter being read out as evidence to a standard Disney-style "we will wipe you and your family from the face of the Earth" (for copying a story that we copied from Christian-Anderson) - who are th jury going to feel for?
It's not about lawyer's being smart, it's about most people being vindictive asshats. If someone is willing to infringe on your trademark it's most likely that they're not willing to roll over and take any polite request to stop. If you send them a letter asking politely for them to stop and they ignore the letter you then have to send another (serious) letter, which is double the lawyer time.
Sure, in a few cases like this one in makes sense because it's not a clear case of infringement and it's more likely a misguided person than a vindictive person, but in more obvious cases (like someone selling "Jack Danielson's" with the same presentation style) it makes no sense to be nice.
So if you don't want to become a fax or xerox or kleenex you have to stamp down on anyone using the name generically.
But you also have to protect other aspects of the brand image. If you can't show that you have rigorously policed somebody using the JD bottle image, then when another drinks maker comes along and calls their product "John Smith's whiskey" but puts it in a bottle like this then they can claim that JD obviously didn't attach any importance to the shape/design of the bottle because they hadn't pursued other users
Second, fax was never a trademark (or copyrighted), its origination is from Latin, 'fac simile'.
Finally, Xerox and Kleenex (and others like Hoover) didn't become generic because they didn't defend their trademarks, the problem was never that rival brands named their products using these names. If I buy a bottle of Pepsi and call it "a coke" then Coca-Cola can't raise any legal objection against either Pepsi or me, and it is this sort of thing that might (and to an extent, already has) lead to other brands being called "coke".
Coca cola famously won a case preventing others calling themselves "Coke" but lost the Cola part when it was shown that they regarded it as a general term for any similar drink. I suspect Apple's lawyers are more careful when anybody else tries to call a tablet a something-PAD.
Coca-Cola can prevent other companies from calling their products "coke" - but there aren't companies out there calling their products "Xerox" or "Kleenex". Their problem is that consumers refer to other products using their brand names, so the fact that Coca-Cola have preserved their trademark is irrelevant if there are people (and there are) who see a bottle of Pepsi and say "I'm going to grab a coke".