I think different sections is overkill. Listing it in the details - definitely a good idea; and maybe a simple overlay on the thumbnail. IIRC, it's been suggested before.

The thing is, we're dealing with IP here, not a loaf of bread, it seems to me but IANAL People retain rights and can impose conditions. I know that it goes against the instinct that says if I "bought" it or it was "free," I can do with it as I want. You can quibble about the use of the word "free," but I don't think "demo" is necessarily more accurate or less confusing.

Personally, I'm a lot more peeved at not being able to sell software that I "bought" than I am about paying for commercial rights, but that's the way the law works. For that matter, I'm more irritated by morals clauses dictating what kind of image I can produce. I always honor requests in that regard, but bypass items that demand it. By the same token some people request donations for commercial use. I certainly don't have a problem with them requiring payment though. If you're really concerned that "free" shouldn't be contaminated with money then pass it on and don't make money on your use of free material. At least offer to donate the profits to some children's charity; then everyone wins.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken