From a business standpoint, the lesson is something I tell my clients all the time. Litigation is expensive so make sure you're launching a battle that's worth winning. In this case, the Coldplay song and album have been massively successful, and just got nominated for a total of 7 Grammys, including Record of the Year, Album of the Year and Song of the Year. This is definitely a pie worth having a piece of. If the record had sold a few thousand copies and then died, I imagine Joe's lawyers wouldn't have done much more than write a nasty letter, but when the stakes are high, the big guns come out.
It's a good lesson to learn and understand. Lawsuits are not cheap or fun. Before going down that path, make sure you can afford it (both financially and emotionally), and make sure you're chasing a prize that really warrants the risk.