The warranty covers the lock - NOT what the bike was locked to. If you're dumb enough to lock your bike to a chain link fence that I can cut with a good pair of snips you got what you deserved. You have to be careful as to what you're locking to - not just what you use to lock up with.
Having said that I only use a cable lock alone when the bike never leaves my sight, ie: stopping off at the BBQ stand on the local Rail-Trail and I'll be sitting next to it VERY soon.