A bit strange as well. - considering the popularity with imac's.
Not at all.
iMacs, like Macbooks are considered "consumer" machines. Mac Pros and Macbook Pro's are machines designed more for the working professional, or serious amateurs, and hence they have considerably more expansion potential and features.
UAD cards are not the kinds of things that average consumers buy, they are professional items. And not only that, they don't make a FW UAD card for a number of reasons, and as FW is the only way you can add things to an iMac (apart from USB), then if you want to be able to use these things, an iMac is not the best choice of machine.
So there is no way to use UAD with imac?
As I already said, there is no way to use UAD with iMac.