I could imagine that some content creators mistakenly choose ARR to mean "don't redistribute". However, if that's the case, they must not realize that "All Rights Reserved" does not mean that and it does not work that way.

And I think you may have a point about "All Rights Reserved" being the default Usage Rights option. But, instead of it being a case of a content creator being lazy, I think it's usually either (a) a misunderstanding on their part or (b) a simple oversight.

For some, it could be an oversight. ShareCG is far from the most user-friendly site I've used. Also, it is possible to forget to set/change something (like Usage Rights) before the 'Submit' button is clicked.

However, I know for a fact that some content creators do not understand the full implications of choosing "All Rights Reserved" as Usage Rights. They do not understand that merely posting it on ShareCG does not grant anyone any rights to do anything with it beyond what the Usage Rights and ShareCG's User Agreement states.

I've communicated with some about this issue. Often, they explain that they chose this because they wanted to protect their ownership. They were afraid that choosing anything else would allow unscrupulous people to resell their content on another venue (such as through SecondLife's marketplace or as part of a 3D package, etc, etc).

Sadly, many content creators do not take the time to fully read all the fine print in ShareCG's User Agreement. If they did, they would understand that they retain ownership of their content, regardless of what they choose for Usage Rights.

(That said, I feel uncertain about "Unrestricted use". The name implies that it grants a lot of rights, with wide-reaching implications. But exactly what this entails is as clear as mud as I haven't found an explanation of what it means.)

My point is that it would be illegal to claim someone's content as their own or sell it without their permission, regardless of the situation. And choosing "All Rights Reserved" - or any other usage rights - is not going to deter someone who is determined to break rules and/or break the law. You might as well put a "Stealing is forbidden here" sign in your store, for all the good it will do you.

However, the very fact that this was published on a site like ShareCG could be used as evidence to support a claim of ownership to have some site remove content that is rightfully yours which someone else is trying to sell. (I'm no expert. But I would imagine that uploading to two separate sites - such as on both ShareCG and Renderosity's Freebies area - may help strengthen such a claim.)