Jobs has stated repeatedly that Apple is a hardware company (vs. Microsoft - a software company).
If Apple starts selling their OS to other companies like Dell, they would get destroyed, especially now that they're on the same chip as Windows.
Why would they get destroyed? Because a magnetic latch and a built in camera can't compete well with the discounts, sales, coupons, and all around low prices of Dell. Charging $150 more for a black macbook would be suicide.
Look at the plummeting costs for laptops these days. Already, the price of notebooks has dropped 18% from last year. Apple's prices have stayed the same, but performance has improved, but if they lost their ace in the hole (OSX), their prices would be forced down and they'd struggle to make a profit.
As it is, OSX is a selling point for Apple hardware that gives it an edge over Dell or HP etc. because the rest of it (with the exception of the slot loading drives) is just interchangable hardware that you can find in any other PC. Take away OSX, and you're operating on gimmicks like the MagSafe, the MacBooks magnetic latch, and the built in iSight. If you go to the Apple website and look at all those reasons you should get a mac, most of them are derived from the OS, NOT the hardware.
gwangung was right about the revenue distribution. Most of it's hardware, so being forced to drop hardware because they're selling software would mean a huge hit in profits and a huge step towards becoming Microsoft Jr. Remember Gateway was making the top of the line monitors awhile back but couldn't sell them at a profit, so they dropped them. I'd rather not see Apple go down this road, and thankfully, i doubt they will.