Most of the Antivirus company are providing their products as a free trial from 60, 90 to 180 days. For a proof you can check here:http://btechreviews.com
They do so that people will try their products and stick on it or buy it. I recommend Comodo to do the same to increase their sales rather than providing a free internet security software.
What I mean is don't provide free Antivirus but provide them for extended trial so that people will be encouraged to use it and buy it.