Why people upload bad CC?

Quite often they don't know they have - The common bad 'hidden' files copy themselves to new Sims created in CAS (Create-a-Sim) even when the Sim isn't wearing them. They cant be seen in the preview shots, and so they continue to spread very easily

If you have ever shared Sims on the exchange while having the typically known bad files in your game, it is very likely you too have spread them.
You can check your own files by downloading them and opening them with Custard or add the links to this post and I will look at them for you. Or you can email them to me peoplewatcherblog@gmail.com

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A Sticky CC theme:
The files in the launcher image have things in common. Opening them in Custard you can see they are in all ages including baby, all clothing types including; pregnancy, Naked, Singed and all career paths and levels.

Files that are made all categories included Naked mode tend to be the sticky ones. Or hidden ones, if you like. I am guessing here but I think the game mistakes them for genuine nude files that Sims have for showers and baths. They are located in the game but hidden from the average gamer. 

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Other strange goings on
I had a Sim loaded with four bad cc files. I isolated one piece of clothing at a time to see what happens. When I removed this skin 'front bum' the four files stopped sticking! 

I played with that skin and countless times BUT I have several copies of the skin Front bum and when I loaded it in to the game via a clean Sim who just happen to have that skin, this DID NOT make those same sticky files stick..

There are other skins that do this too, but I doubt it is the skin itself at fault and believe they are safe to play with. If you have one that is attaching sticky files, the only way to remove skins is to delete the Sim it came with unfortunately. More advice on that here Removing CC

The best advice I can offer is DO NOT download whole Sims. Download individual clothing and other items from The Sims Resource. Its' hosts oodles of free stuff, files are sims3packs (visible in the launcher) and all clearly marked how they are categorized.