First, I don't see zip-lining on that itinerary, so I'm guessing it's an optional bonus?
I don't know about loss of limbs, but I can see how someone who did not pay attention to the safety instructions could lose a finger on a zip-line tour. It is always prohibited to be under the influence of alcohol or drugs, and you must understand that you are undertaking a risky activity and PAY ATTENTION to the safety instructions.
It's always best to check out the company and its safety rating before going on adventure tours. That said, I *believe* SkyTrek, which operates out of Monteverde and Arenal, is a reputable company.