I'm going to need more than that, I've never seen GPL code needing a commercial license.
You're right about the code, but just because something is released under GPL because it has
some GPL code in it, doesn't mean all the code is free. If they also use patented code along side the GPL, then the patent holder has right to sue and require royalties and special licensing for using their patented code.
Hell, there has been a "GPL" exFAT driver available for Linux since way back in 2013. Haven't you wondered why it hasn't just been included in the kernel a long time ago? Because just because it is GPL doesn't release anyone that uses it in a commercial product from the liability of using Microsoft patented code in their products.
Now, with the latest development that Microsoft has released all the patents to the OIN that issue goes away, but that just happened a few months ago. And it takes time to write software, test it with your products, re-write your product so it will use that new driver, etc. Plus you have to bring in a legal team to make sure that Microsoft is actually on the up and up and isn't setting up some way to come back after you later down the road.
Ok, that makes sense. But it won't work in the long run. I mean, they're not probably shipping with the latest kernel, but what are they going to do? Never update to a kernel with built-in exFAT support? Update, but gut that support? Both are possible, but neither is likely to be worth their trouble.
Once they do enough testing with a kernel that has it built in, they'll probably release it as an update and all will be well. But for right now, it's $4 because the kernel they are using doesn't have it built in for free, and they still have to pay for the commercially developed solution they are offering.
So you have a choice: carry around a USB drive around the house or pay a premium for something that exposes that data over the home network.
At the end of the day, I don't really see why not having exFAT support on a NAS is a big deal.
The benefit of exFAT is for large external drives, and that's it. Ok, so you have something on your flash drive that you want to transfer to your NAS quickly. You can either walk over to your NAS, plug the flash drive in, then go to your computer and do the file transfer, then walk back to the NAS and unplug the flash drive. That's stupid. Just plug it into your damn computer.
But then they'll say "but what if I want to just leave the drive plugged into the NAS?" Then format it to something else! There are 2 main reasons to use exFAT on removable media like a flash drive or external hard drive. The one is exFAT is actually really resilient when it comes to being unplugged during a write operation, it is possible to really screw up an NTFS drive if you do this, but less likely with exFAT(and FAT) and it uses slightly less resources on the computer/NAS during file operations. Ok, if you are leaving the drive plugged in, then the first reason goes out the window. The second doesn't really matter, because the difference is negligible on a NAS device. So if you are leaving it plugged into the NAS, format the drive to something else.
I mean, I can't really come up with a scenario where a NAS even needs exFAT really.