Not a clue as I'm not a developer. That's also for FreeBSD 11, so it may or may not even be compatible with 9.3.
Ultimately there's a few reasons why USB3 support being absent in FreeNAS isn't "that" big of a deal in my opinion:
1. It doesn't affect bootup times, shutdown times, or server performance.
2. USB as a data transfer medium from hard drives, is just not reliable for a bunch of reasons. Some are hardware based, some are real-world situations that can't be fixed with engineering, etc. We've had so much data loss from USB that we don't recommend it, at all, except as a boot device.
3. There's a limited amount of programmer's resources available, so the devs can focus on things that are very important and do affect things like server performance and reliability, or they can focus on things that don't affect 99%+ of FreeNAS users (and those users aren't going to use USB for data drives because the same 99%+ of users know that USB is a bad idea for a file server).
There are some tickets out there on USB problems. You are welcome to ask if the devs know about that fix. But they probably know about it already. :P