There's a few of these threads about it. It's been an issue since day one. For now, the only real preview available would be YouTube searches and the like.
It'd be a good bet that most, if not all, of the songs that make it to the PS3 probably have some kind of video available. But, not all songs on the 360 have one. For example, I'm not sure if I've seen a single a t o m video.
In this case, it's just going to take a little more effort to get a preview. Perhaps it will be fixed when RB3 comes around.
RBN - You're Not Alone - Rock Mix - Drums. The quality of that tracks makes me more interested in the rest though.
You know, it just occurred to me. If you really want free songs, buy songs in packs rather than as singles.
I bought the Ozzy Osbourne pack yesterday. There are six songs at $2 each, so $12 for all six separately. However, the pack was $10, so I essentially got a song for free.