Because they were hoping to get it cheaper than the inevitable legal fees will now cost them and Cisco. I bet the negotiation went something like this:
Apple: "Look, we can pay this much for it, and we'd rather you got it than our lawyers and your lawyers. Sure, you can sue us, and you might think you'll win. You won't, but ... like we say, we'd rather you got it than lawyers."
Cisco "Sue? Ooh, we get to defend a tenuous trademark! Hee, hee! We'll sue, cos what you offered us isn't worth the jackpot we'll get if we win. Ha ha ha! My Brother is a lawyer and he needs a new Maserati."
Basically, they're not suing, they're negotiating, in court.