It's actaully nothing to do with Bethesda being money grubbing, it's because of their issues with PSN, mALX could explain it a lot better than I can, but she's not here so you'll have to make do with me.
Sony charge developers for all DLC they put on PSN, whether it's free or not, Microsoft don't do this as they charge their users for Live, there are also some incompatability problems with the PS3, the 360 is similar to a PC in terms of architecture, while the PS3 is not, this means they have to code a lot of things again for the PS3 versions of their games, and with the already mentioned cost of putting DLC on PSN, it would be time consuming and costly for them.
I'm not entirely sure about the specifics of all this, as I said mALX would do a much better job of explaining it.
And lastly, I am in no way, shape or form trying to start a squablle here, I too wish the DLC was available to the PS3 owners, I'm just hoping to help shed a little light on the issue.