It's important because money is involved, basically.
If you're trying to advertise and encourage people to buy stuff, it makes sense to know who's going to buy things and how they're willing to buy them.
I like buying things on my iPad or iPhone from sites like Amazon. The experience is good and there's not much hassle. I spend almost no time at my home desktop machine now that I have an iPad, so if Amazon puts more money into the iPad user experience, it benefits people like me and apparently has a return on investment because I'm not alone in being willing to buy stuff from them from my mobile device.
It's also interesting. Why is this happening? Is it because there are people like me that are now 'post-PC'? Are those people more likely to be iOS users? Also, I bought my Mom an iPad mini last Christmas, and I bet a lot of other tech-ish people did the same or similar just to save themselves tech-support hassles. So people like my Mom that aren't super good with tech might now be doing shopping on their favourite web device, and those people may disproportionately fall in the iOS camp.
It'd be interesting to know. Are you really not curious at all? It doesn't have to be an iOS vs. Android thing, really. There's clearly a dichotomy, but there's no value judgement to be made unless you're really invested in being partisan about it.
In the end, this benefits Android users as well. This is more a matter of who to target first, but a properly constructed mobile site is good for everyone with a mobile device.