A way to look at it is by pointing to a popular mainstream movie called “Zack and Miri Make a Porno.” It is here and other instances where you can see that porno refers to a single movie or something similar. Porn on the other hand, is more general as it is non-specific. More so when it refers to the quantity of it. A person can have all kinds of porn on his or her computer. They can also have one porno movie. Still, others argue that the word porno can be used in a general sense to mean more than one.
Regardless to how people feel about it, there is one thing that most can agree on. That is the fact that the internet is full of free porno or free porn. Either way you say it, the result is the same. In the end, people get to see all the complimentary pornographic material that they want. Porno is always an adjective which some feels refers to smut from the past. A phrase that is anachronistic and retro in its usage and pronunciation. Then you have people who use it interchangeably and to fit whatever they are trying to say.