It's not from a virus scanner perspective, but because of how to implement it.
Specifically, the less that the kernel driver for the pool has to do, the better. Checking for extensions for each file that accessed is not insignificant overhead. Especially when dealing with file modifications.
As for antivirus on the server, it absolutely depends. Any antivirus not designed for it can cause issues. For instance, the client backup database is raw data, and likely to trigger false positives, just based on probability. Additionally, it can interfere with the access and corrupt the database.
Additionally, because it is a server OS, a lot of software will just outright refuse to install, requiring you to purchase a license for a server variant of their product.
But generally, you want antivirus software installed on any system that is accessing content from the internet, to prevent any malicious programs from running on that system.
And to clarify, while this isn't something specifically we've considered already, the principle for it is. It doesn't really fit cleaning into StableBit DrivePool, and more so, we don't want to squeeze a bunch of things into StableBit DrivePool, especially if nobody will ever end up using them.
That said, it is more in line with something else that we'd like to do as a separate product.