They don't have to 'force' it, they just have to make it very expensive.

ARM has a three-tier licensing model (http://www.arm.com/products/buying-guide/licensing/index.php...)

If you believe the wikipedia entry on ARM licensing costs, (http://en.wikipedia.org/wiki/ARM_architecture#ARM_licensees), then the average cost (in 2006) was $0.11 per core, averaged across all cores.

At at 50M iPhones, thats $5.5M (USD). Even if Apple is paying $2/core in licensing costs (effective, the A4 is the first non-Samsung part used by iPhone OS), thats still $100M, so $8B doesn't pencil out.

I do find it incredibly likely though that Apple would 'buy out' their license(s), and negotiate to have a minimal (maybe even zero) cost license to all future ARM IP.

Or they could just buy a controlling interest (probably wouldn't take 51%) and frighten off everyone who competes with Apple.

Just the slow-down for the rest of the market alone (Atom isn't anywhere close to ARM in terms of functionality/watt), it might be worth it.