They probably don't want the grey market to flourish, where Indian MacBooks sell for $700 locally and get shipped to the US in an eBay transaction. Geo-locking a laptop in the modern world is impractical if not impossible and would have its own PR issues.
There can't be a grey market if you charge the same price globally.
I think geo-locking is definitely a workable solution. Add a cheap GPS receiver and the machine can tell where it's located. Then if it's moved between countries, it can refuse to work until the user pays up. Since Apple controls the OS, they can bake this in: bring up a web browser page pointed to a payment form, linked to their Apple account.
I don't see why this wouldn't work. It's not like Apple users are going to rebel and refuse to buy Apple products because of something like this.
There can't be a grey market if you charge the same price globally.