The first reason is that a $99 cashback may well cost less than that, simply because some people will never get around to it, or some fine-print in the terms will be able to exclude them.

The second reason is presumably to get some demographic information about who is buying the machines. Once you've bought the machine, you might be prompted for all sorts of information in order to request your cashback, and since you're being 'paid' for it you might be more willing to answer those questions.

Certainly I know that I have never registered a computer before now.