I'd call it just plain old bad luck in your case. Move on, or even move up if you think that will better suit your needs, but valve train failure is one of the least reported issues on here.
The big elephant in the room for me would be the exact nature of the failure. Did the valve sieze in the guides and bend with the springs and keepers intact which would indicate either an oiling or valve float problem, or did the keepers let go and the valve simply hammered the piston? These are very different critters. One would indicate a possible clogged oil passage or use of crappy oil which could bite you again down the road, one would indicate sloppy attention to valve adjustments on your part which you can correct in the future and one is just plain luck of the draw.
You don't wanna go through this again. I'd ask.