That's dumb.

I don't have xbox live.
Yeah, Silver is free. I bounce between gold and silver all the time, depending on what games have come out. Canceling your gold is a bit of a pain though, you have to actually call MS support (The process itself is pretty easy, but the fact that you have to do that is lame) Their reasoning is apparently lots of angry Girlfriends canceling boyfriends Gold online or in the dash, so those were removed.
But no, you don't need Xboxlive gold to buy stuff. You don't even need to submit any payment options. You can go right to a local retailer and buy the point cards, which is usually cheaper because they don't get taxed (Online purchases are taxed, you save a whopping $1.60 in California)