Well basically you have to look at it logically. When something has funding behind it, they can afford to hire more programmers. They can afford to regularly update their virus definitions (Finding new viruses), etc.
Money runs everything. Something given away for free usually can't keep up because it doesn't have the funding to compete. Sure it has virus definitions, but they're outdated and not as useful as paid versions.
I'm not saying all freebies are bad. Spybot Search & Destroy is a great example of free software, but never stand alone. It's a great spyware and malware detector, but because it's free you don't want to use it alone. I used to use it with Ad-Aware: SE-Personal. Something like that. AVs are a bit different though as they can conflict with eachother if both ran together. That's why it's either go big or go home when it comes to AVs.
ANother really good example of freeware software is Zone Alarm's free firewall. The reason this is exceptional outside of the rule of "can't keep up if not funded" Is because they sell paid products. Their main juicy firewall costs a shitload, so they're able to cut off a few features and give away a lighter, not-as-good version.
Basically, TL;DR: Most free products (Company made, not user made) are shit, not because they're bad themselves, but because the gap between them and programs that have funded dev. teams is too great when you can turn around and torrent the better stuff.