ClamXav is no longer free. The version offered through the App Store is the older 2.6.4, which is free, but not up-to-date. You can then update to the newer version 2.8 or higher, which is a paid version. You could call it an in-app purchase, I suppose. If it is important to you to have up-to-date antivirus software (what's the point otherwise 😀 ), then you will get to pay for ClamXav - or use something else that remains free. (I expect that those remaining free options will also be gone soon)