They are doing this to compete with diapers.com. Everything must be in new, unused condition to return it or they won't accept the return. You can take it out of the box and try it in your car, I'm sure, but obviously they can tell if it's been used. (Think about it like ordering a pair of shoes from a site that has 365 day returns)
The biggest risk they face with carseats is losing a year on their DOM, and honestly, I don't think they much care about that (and sadly, most consumers don't know or realize about DOM).
FWIW - I have ordered car seats and strollers from diapers.com and I order lots from both amazon and diapers.com. I have returned items to diapers.com (never a car seat though) and I think the longest I've ever taken to return anything is maybe 2 months. I can't imagine many people wait a year to return anything. It's just part of their customer service pledge