They sell it as a "convenience". If you just want to do all your shopping with them, they'll charge you for the convenience, because it's inconvenient for them to stock RAM. They know full well that you can get it cheaper elsewhere, and would just as soon you did. The upcharge is basically a penalty.
Not a big deal, really. My company - and pretty much any manufacturer - will do the same when it's not their main line of business. It's really set up as a disincentive to bother them with minor generic upgrade parts. But, if you're REALLY willing to give them the money, then...