Yeah unfortionatly i have the same problem. Took my car to several dealerships and now i work at one. They gave me a print out showing it is a known issue that is not under warrenty. I plan on takeing mine out and resealing them. Hopefully it will work. I dont personally know why they let in condensation but your not alone. The techs at my work cant figure it out either(not only why there is condensation in my headlight but why subaru would put out a product like that that isnt covered. Anyway i also had to replace a headlight due to it getting smashed out but i had it replaced with an oem headlight, which unfortionatly also lets condensation in.
However here is the wierd part i think you might find interesting...
This was at a show, the car was in the sun and it spit only a couple rain drops. Those are the raindrops on the headlights but where they landed you can see the condensation that formed in a matter of seconds. It was the only time this happend and i thought that was messed up lol