Apple has a very long history of implementing hardware drivers "in software". Steve Wozniak famously did this for a floppy disk drive for one of Apple's early computers.
I think it's just a different, integrated, approach to hardware a software development. If you're doing things custom anyway, then why add an extra chip?
Because:
1. Software is more likely to fail at protection with worse consequences when it does (fire, damaged goods, warranty claims). Not just now, but also the future updates.
2. It eats away at the resources that are intended for the user. In other words: it makes the machine slightly slower for the user, for no good reason to the user.
3. You can do things that are impossible in laptop OS software. It gives redundancy, even if the OS freezes you can still make sure the speaker doesn't overheat. If it's implemented in a seperate chip. Also there is real time ability, etc.
4. It makes the OS and drivers much much simpler, which is important if you want to support other OSes on the same laptop.
Advantages, for Apple, to do it in software:
1. Software upgrade is easier and cheaper (assuming they never ever fail).
2. Cheaper.
3. You can keep competing OS'es off of your hardware, because it's too hard to write drivers for your secret sauce closed source drivers that include functionality that is "preventing parts from frying themselves".
I think it's just a different, integrated, approach to hardware a software development. If you're doing things custom anyway, then why add an extra chip?