You dont get it. Apple isn't really a software company. They make software simply to support and improve their hardware. They have always been about the hardware. Today, as always, Apple still sells far more computers than Microsoft. Oh, and their market cap and profits are also higher. People will buy the iPad for the "Apple experience". Putting their OS on an HP product would give buyers a hardware alternative to that experience.
Apple has got really smart over the last number of years.
Up until about a decade ago their business plan was: We'll sell our hardware (Macs) by positioning ourselves up against the competitor, who owns 95% of the current market share, but our computers will be so much better built and our software will be so much nicer to use, that everyone will switch from their Windows + PC solution to buy our software and hardware.
Which kind of worked.
Today, their plan is: We'll sell our hardware (iPods, iPhones, iPads) by positioning ourselves in nascent new markets where there is no clear single winner, and our products will be so well made and easy to use that we will arise to be a dominant player in the market. We will offer the premium experience and an opportunity for vertical integration that will also drive sales in other areas (Mac hardware, carrier subsidies, profits from App Store).