wet/dry filters and nitrates

[mikey77]

I'm just curious why wet/ dry filters with bio balls and the like, are always criticised for being nitrate factories and people are often told to remove the media and replace it with live rock. Isn't the live rock basically doing the same thing as the bio balls or ceramic rings? Aren't they both just porous materials in which bacterial colonies can exist? The reason I'm asking is I just got back from a local lfs where every tank has some type of wet dry filtration on it and they all look great, including the all sps display. Not trying to start a debate or anything, just interested in the scientific reasons why a wet dry would cause a higher nitrate level.