Maybe they figure that those buying oem kits know what they are doing and those buying the retail kits don't.
Also, the fact that the retail drives come with their own box and impact-resistant packing probably gives a false sense of security to the retail store clerks who don't cringe if they knock over a whole stack of them The oem drives, on the other hand, I've seen many inserted into a single styrofoam box that had individual slots in it for each drive. I think everyone would tend to handle a box of these oems packaged this way a lot more gingerly haha
My last WD 120 SE drive with 8Mb cache I didn't even get replaced after it failed... Even though the sticker said it was in warranty, online it wouldn't accept it and I decided I have so darn many sub-250GB drives that I didn't want another around, so I just unwrapped another 250GB drive I hadn't used yet and stuck it in it's place. So 5 yrs warranty vs 3 yrs vs 2 yrs doesn't mean a lot to me unless it included free data receovery, which of course it never will. Drives are so cheap - you want peace of mind that it will work upon arrival and last for months upon months, but sooner or later technology passes that old drive by. Under warranty-replacement you always seem to get a reconditioned unit from Singapoore or Ireland or wherever, and the replacement drive always only has a year warranty on it so there is small satisfaction in that. I don't want a refurb with a 1 yr warranty!