I see this "if you don't defend your trademark, you lose it" trope a lot on the Web these days, generally with the implication that the trademark holder has no option other than to engage in legal threats or action.

Couldn't the trademark holder in these situations simply license the mark to the other side, for some nominal fee?

Same for the Jack Daniels case. The book cover is probably fair use through parody, but even so, Daniels could simply license it to the author and request that the next printing have "the cover design is a trademark of Jack Daniels Properties, Inc.".

Sure - up until the point that OK Go got publicly indignant about the lawsuit, everyone acted reasonable here. One party had a problem with an action another party took, so they communicated about the problem to the other party. The two parties weren't able to come to an agreement, so the other party asked for a court to make a decision. Not really much of a story, I think.

Seems more like they were "quietly threatening" because they realize "ok go" is such a generic term that following through is likely to fail.