For one, it doesn't help HP or Apple. There are companies that use Macs you know, and they go to Apple to get them. Nothing is stopping anybody from going to Apple. Also, the discount that Apple gives is quite pathetic; something that doesn't really help them either.
What would Apple get from HP? A better distribution channel? Seems to me that Apple is doing that on their own, in fact companies are coming to them. BestBuy, Walmart, etc. The Mac Mini will be found in stores. If Apple could get Walmart (as much as I hate them) to offer the Mac Mini, that would be a big outlet.
You still miss the issue that companies just cannot just one day say they're going to switch. Training is the biggest reason as is software and testing. Want proof of this? Itanic. HP is pushing that hard as a replacement to its PA-RISC processor. Applications need to be recompiled, guess how HP is doing? Yep, customers are leaving. If you have to change, which they do as the chip is no longer being progressed, you might as well as look to see what the competition offers. After all, it's the same amount of work no matter how you look at it. The competition might even be willing to assist you more then HP is. The last survey of companies that use the PA-RISC or Alpha processor, over half had no plans to switch to the Itanic in the next ten years if at all. A good majority of them are in the not at all category. Very little high-end kit stays installed for that long. It's big money and support lasts for three to five years after EOS and then it moves to EOL status. Companies start looking around the EOS date. Switching costs a lot of money, it's not cut and dry as you seem to believe it is.