There are several reasons for this. First off, my primary book review log is here on Livejournal. I mirror everything here (and a few from elsewhere) to LibraryThing and most of the more recent book reviews are also on GoodReads and, via Facebook, LivingSocial. Both of the latter have their downsides and irritations, but it is pretty easy to copy the reviews across.
I've put several dozen of my reviews on Amazon, but find the interface annoying; no markups or hyperlinks allowed, and the extra bother of thinking of a title for the piece after you have written it. In addition, the social interaction of Amazon reviews is markedly nastier than anywhere else where I have been publishing them. I do not think of myself as a particularly delicate flower, and have no objection to robust debate, but I am struck by the preponderance of negativity in the few comments my reviews there have attracted (a small symptom of a wider problem; there have been several much more dramatic and well-publicised instances of Amazon review meltdown). It's not a pleasant space for debate, and I don't really want my words adding to the vicious spiral.
On top of that is Amazon's unpleasant corporate behaviour. I have hinted it before, but I will say it directly now: I think the Kindle is a swindle. There are already cheaper and better electronic book readers out there, and in particular there are readers with a sane and sensible approach to digital rights management, which I translate into layman's terms as meaning that you actually own the book that you pay for. The recent spat between Amazon and MacMillan did no favours for either side, reputationally, but Amazon were the more obviously evil of the two companies.
I shall probably continue using Amazon, for purchases, because living where I do and reading what I do I have little option (and they do get very good marks for delivery). But I simply do not want my words on their site adding value to what they are doing, so I shall spend some time over the next few days clicking "delete" on my reviews - which will of course remain visible here and elsewhere, as described above.