It is a subtle, but important, distiction that is often used as a rationalization.
Edit: This really belongs on the child comment.
If I'm walking in a forest and I pick up an apple, prior to my picking it up, it did not belong to me. Upon picking it up, it now belongs to me. You can claim things that did not belong to you prior to said claiming.
I get what you're saying but it only applies in situations where everything is already owned by someone.
My take on Tactic's explanation of stealing is that the guilt you should feel from stealing should also be from having taken something that isn't yours from someone, instead of just the guilt of depriving someone from the thing you stole. Although both perspectives are good to have, taking something that isn't yours is a better framework to think in so you can avoid the grey areas of stealing.
I agree that this is likely what he meant. It's a much easier way to get children, who likely are too young to have a complex moral framework, not to steal or even come close to it.
However, we're also talking about adapting that lesson to adults, who should have that moral framework. By maintaining the black and white view, you're depriving yourself of the full use of your rights, or potentially contributing to the denial of someone else's.
Children actually innately understand some of this; if you're not using your blocks, why can't I use them? In a more adult sense, if you're not using your land, can I use it?
It's a bit of a tangent, but I think it might be just as important to impart the lesson that you should use and maintain the things you own or claim, or potentially be subject to their loss. There are significant advantages to understanding the communal claim on many types of property, and they don't just apply to individuals but the community as a whole.
(edited for clarity)