The differences in warranties between most indoor APs and outdoor APs is driven by a combination of the harsh conditions where outdoor APs are deployed and the fact (as alluded to in this thread) that there is a high-risk of failure due to improper installations. The APs themselves are very robust and generally have a low failure rates when installed properly - nevertheless they are prone to higher risks due to inclement weather conditions or bad installations therefore they carry a 1-year warranty.
Thanks,
Will