When I buy a product, I don't feel obligated to help the vendor sell more everytime I use the product.  What if I did a horrible job using the product?  Would I also then be in liable for possibly costing the vendor more sales?    If I buy a model and use it in my image without crediting the modeler, that is not plagiarism.  If I buy it, use it, and state that I created it, that would be plagiarism.   When you go into a gallery to look at paintings do they list the manufacturer of the paints and canvas along with the artist's name?  Do you assume the artist created the frame, if a frame manufacturer isn't credited?  How about the brushes?  Some artists do create everything themselves, but these items are all tools and materials used to create the work.  I'm sure those companies and their employees make a living off of those sales too.  By the same token, do you have oodles of signs in your front yard helping to advertize all the tools and materials used to build your house?  Now there are times when folks will advertize the products they use, but typically they are compensated for doing so.  At the very least, they get the product at a discount or for free.   I think that's why many will credit free stuff items when used.  You got it for free, so you give the creator a mention or a thanks in exchange (even if they didn't ask for it).  A vendor gets his/her money instead, because that is what they ask for in exchange.  Anything beyond that, should be considered a bonus by the vendor and not an unwritten requirement.  If I'm really happy with an item I bought, I'll credit the vendor.  The vendor should be pleased that I am noticably happy with my purchase instead of being angry or feel slited by those who didn't credit them.  Perhaps it should be taken as a challenge to create something that will garner more praise (and free advertizing) by their customers. ;-)   

Irene-