Is that it's not an exemption, it's simply a different scenario altogether. Nobody likes having their employee take longer than expected to do a job, it's true. But Martin isn't your employee, nor mine. In terms of A Song of Ice and Fire, if he is employed by anyone it is by his publisher. If any kind of contract for services expected to be rendered exists, it exists between him and them. We have no legal part in it. If he takes a long time, it's up to the publisher to decide what to do about it; in most cases, their choice is to encourage/prod the writer and publish the book when it's done.

Even in that respect, they are not like a typical office worker. Authors are in most cases more like freelancers. You offer to pay them for a service, you get an estimate for how long it will take, and then you pay when the service is rendered and complete. If it takes longer than the estimate, you either terminate the contract and hire someone new (impossible as Martin owns the copyright for his characters and world) or you wait. Maybe you get angry, but you wait.

The writer will suffer financially if he takes a long time to write the expected book, because he doesn't get fully paid until it's finished. But again, that's up to the writer.

I was a newspaper writer for five years, and there you are expected to meet deadlines (or else), but that's different from an author. The newspaper could fire me. The publisher isn't going to fire Martin, because they're still going to get their profit eventually, and they don't have to pay until the material is ready.

If the publisher (or anybody) was paying Martin as he wrote, an hourly or weekly or monthly salary and that was all he got, then he'd be like a regular employee. But they don't pay him like that, and nobody else does either. All he gets are advances, which are subtracted from his final payments when the book is finished.

The key distinction is that authors are paid when the work is finished, not while the work is taking place. So if the author takes longer, the financial burden is on him. We, the consumers, have still not paid a red cent until the book is finished and published. We may be somewhat similar to shareholders (if enough of us stopped buying the books, there would be more pressure on him), but we aren't his CEO, we aren't his managers, we aren't his employers.