I also was confused about the pricing policy but after I posted on their forum the problem was clarified. The software is free but you have an option to pay for it and you get the following advantages:

High-speed updates
Dedicated 24/7 support
Phone support

Also the free version includes unlimited updates (both virus definitions and program updates).

This piece of software is pretty cool if you ask me, I gave it a try and was very satisfied with it. Very light on system resources, good detection rate (as far as i've tested it), very simple interface and very easy to use.

Now the problem that remains is: Do you really need an antivirus for the mac? As I saw on a forum, someone said: the best antivirus is the one that sits between your ears :D. Especially on a mac ;).