Now that I've had a chance to put my head back on, let me respond to some specific points.

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these friends made back less than half of what it cost them to host the function

The notion that you expect anything in compensation for the wedding is patently ridiculous in my culture. Showing up without a gift at all would be slightly rude, but not by a lot. The bride failing to send out thank-you notes would be a far worse crime.

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Many people at their work were "offended" that they would expect cash as a gift. WTF! Have these people never been to a wedding in this country that wasn't some anglo-trash cold-cut and beer setup?

White trash is about the only group that I can think of as far as "regular Americans" go where cash would be an appropriate gift. And I'm not even sure of that. Period. End of story. Honestly, I doubt that many people these days would be terribly offended by cash as a gift, but their parents definitely would be.

Think of it this way: if someone invited you to a party, would you bring cash as a hostess gift? Why not? To me, the same thinking applies to a wedding, with the additional assumption that the party wasn't really thrown by the bride and groom to begin with, but by the bride's family, so you're really giving the compensatory cash to the guest of honor.

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personally, I wouldn't think of attending someone's wedding and giving anything but cash unless they specifically asked for something else instead, like via a registry for instance, and mentioned something about not wanting currency.

In polite society, the notion of not having a registry is unheard-of. If there's a party big enough that you even begin to be worried about the cost of it, then there's a registry. Buying off-registry (unless you're very close friends or family) is somewhat insulting. And I can guarantee that cash is not on the registry. As with all gifts of cash, it implies that you didn't bother to think about what the giftee wanted, compounded by the fact that there was an easy-to-access, public list of what was wanted.

I will point out that my culture is based in the vaguely upper-class Southern US, which certainly has a bearing on my viewpoint, but I don't think I'm out of line with most of polite society in the entire US. (Well, at least in what I'm saying.)