I've dealt with this issue in the past and it's really a no-brainer.

In the real world, licensed products exist everywhere.  Many have restrictions with their license.  Sometimes, those restrictions are included because they are legally necessary.  For instance, there are plenty of products that can not be exported to certain countries outside of the United States.  Why?  Because, they may contain technical advancements that could aid a hostile nation.  This is "THE LAW" and not some whim of the creator.  Yet, any idiot can see that a cell phone computer chip IS going to find its way to someone that the Law would seek to deny.  But, the Law is there anyway, in order to provide recourse for prosecution.

Sometimes, restrictions exist to protect the economic interests of the creator.  This is very common in software.  Just because you can copy it and install it on 100000 machines, doesn't give you the right to do so.  You must buy a site license to copy one piece of software on several machines or, at least, purchase several registrations.  The OS that you are running, right now, is, most likely, bound by these restrictions.

Lastly, comes art.  For years, artists have been giving away pictures that have commercial restrictions.  Why?  Because, if someone wanted to they could just collect all the free pictures on the net, put them on a website, and sell them.  This is a) Counter to the desires of the creators who produced the free artwork for personal use, b) Allows another to personally profit from someone else's labors without any acknowledgment of their contribution and lastly c) an Artist lives entirely by their reputation and that is threatened when they are unable to receive credit for their own work.

Look at it this way:  Let's say that the game "Monopoly" was published by Parke Bros and offered to people for free so it could be play tested.  A competitor sees the word "Free" and grabs 20,000 copies of the game, puts a sticker with their name over Parker Bros logo and then sells the whole truckload to Toys R Us.  What is Parker Bros recourse if they did not have license restrictions with their product or were not otherwise protected by copyright law?

A true, commercial artists knows copyright law and how it effects their own work.  They will do all that they can to avoid treading into the realms of the unknown by using spurious leavings on the net without researching the licenses.  Another above said, (paraphrased) "Embrace your hobby and don't pose as a professional."  A professional will research the licenses before-hand and will not arbitrarily download work and use it willy-nilly in commercial products without paying specific, professional attention to this.  An amateur will not.

The creator has a right, by Law, to limit the use of their work.  By downloading that work, you agree to its restrictions of use, within reason. (Content use may not be as easily enforced because it is difficult to define but commercial use is extremely easy to define.)  That is the bottom line and no amount of "I can download it and that must mean I own it completely!" will change that.