So check it. By grabbing copyright on a song they never owned, Warner has made at least 47 million dollars. Sure - $2m a year is nothing (chump change! pick it out of the couch cushions!) but now all of a sudden WB will have to give it all back. With interest.
This is money they've gotten without even having to ask, really. It's just known that "Happy Birthday" is expensive, toxic poison. I work on a major network show and one fine day, live, one of our contestants busted out into a rendition of "Happy Birthday." We had to cut Warner a $150k check. It wasn't even a consideration. It's just what you do. Because, you see, copyright is monolithic.
Which is why, when I applied to Fox for a license to "Are you lonesome tonight" for a film (to cover it, not to use the Elvis song - I'm not crazy) I just accepted it when they told me to pound sand. The answer wasn't "you can't afford it" the answer was "we will not have you besmirch the legacy of Elvis with your scummy low-budg horror film." Never mind that Anne Murray covered it. Never mind that the song was first published in 1926... without copyright.
'cuz see - if Warner loses this, I get to cover Elvis. For free. And Fox can fuck off. And that's just one of many cascading probabilities from having David slay Goliath on this one - the giants will come tumbling down.
You'd fight it tooth and claw, too.