Artificial intelligences are created in a virtual environment, where they are stored in a "creche" of other AIs in their generation. When bootstrapped to self-awareness, they are given a choice of function- commercial use (AnthroPCs), military, scientific, etc, or allowed to subsume in the global meta-AI. If they choose to go into "retail" they are allowed to choose a self-identity and are shipped to a reputable "dealer" (such as Idoru, in today's strip) where they are put up for "sale."
Purchase of an AI is not a binding contract- either party is free to terminate the relationship at any time, and the transaction agent will refund the contract fee. The use of terms like "sale" and "owner" are considered offensive by some, and are becoming rather politically incorrect (my use of them here is solely for sake of comparison, hence the quotation marks).
One would think that the majority of AIs would choose to simply be given a chassis and left to make their own way in the world, but the majority who do not go into a specialized profession choose to pair up with a human "owner." There has been much speculation by both humans and AIs as to the reason for this- no aspect of their programming indicates a cause for such a bias. The general consensus is that the average AI simply finds your average human entertaining, and enjoys the companionship.
tl;dr I am a big ol nerd