There are lots of Free Antivirus out there e.g. Microsoft Security Essentials and AVG, so why should your business not use these to save money?

A very good question we can answer

Lack of centralized management

When managing over five computers you want to know what machines are up-to-date, What machines have had infections and where these are, and what machines have their AV enabled. This is the best way to mitigate a wide spread infection especially with Ransomware , where the last line of defense is disconnecting a computer from the network .

Licensing 

A favorite of ours for our home users is Microsoft Security Essentials. It's great on detections and it just works , howver it is for "home" users. Microsoft actually do not allow this for use in businesses for over ten devices. https://support.microsoft.com/en-us/help/13752/windows-security-essentials-eula

Free? 

Free Antivirus such as Panda Free and Avast Free make money from you by installing third party applications during install and also changing your default search engine and home page so they can monitor your browsing history and target adverts at you. AVG will also notify the user with deals to upgrade to professional version which waste's time.