There's nothing in the rules that explicitly says that.
The description for the Ported Icons section reads:
This is logically where people would go for other games' icons converted to work inside Warcraft 3 but otherwise unmodified. Not icons for existing Warcraft 3 resources.
In the Site Rules it's expanded as:
There is also stipulations that:
My icons for this and the Darker Night Elf buildings are neither Warcraft 3 icons nor from another game.
'Original work' doesn't begin and end with drawing a line with your mouse. Honestly this Nature Guardian icon isn't a great example of this (I don't think it's my best work but I feel lit has a certain something) but the Darker Night Elf Building ones are- I did a bit more than just screenshot. I used filters and used the World Editor to get an angle and lighting that would make for a good screenshot to use. I used the brush tool to emphasise and de-emphasise certain parts and change the lighting. Even though I didn't go as far with my work on this nature guardian icon, I did still do more than screenshot.
Why not just put them as useful/simple in the main items section? I'm not trying to be like 'you have to like my work' but it is frankly insulting and tiring that they keep being sorted into categories for 'unoriginal content' when they do involve artistic work and decision making. I uploaded these because I thought they'd be useful to people who wanted to use the resources they're based off of, and ported doesn't seem like the place people would go for icons that fit Hive Workshop resources.