That stems from the "in whole or in part" of the law.
I'm an artist and I sell my original paintings and prints. Just because you buy either does not give you the right to copy any portion of my art for any reason without my express permission or I will sue your a$$ for stealing from me. If you want to use my art in your website, there is a good chance I might let you do this. If you want to copy my art and put it on a t-shirt, hmmm, maybe not. If you want to copy the original art you bought, put the image on a T-Shirt and sell it, then be prepared for a law suit because you are stealing from me. Just because you bought the art (or print) no other rights are confered to you. All rights to the image belong to me unless, you and I enter into a contract and you pay me for those rights.
You can't sue anyone for stealing from you because you have lost no physical possession.
I've always been of the opinion that transfer of the original should by default concede all rights to the new owner, but Copyright law is the opposite, it defaults to only transfering the physical entity, and not the Copyright, unless explicitely given. When I do commissioned works, though, I always (simply because I personally feel it's right) give rights to the buyer, except that I retain a non-exclusive right to use reproductions for my own promotion, e.g., to include in portfolios.
That said, if you would sue someone's "a$$" for putting a portrait you did of someone on their funeral programme because you own the rights and technically they weren't allowed to reproduce it, you're utterly heartless. Yet, it is in things like this, programmes for weddings, funerals, birthdays, etc, that the most common Copyright violation of plastic arts occurs.
As well, unless you routinely register your work with the US Copyright Office (which most artists don't), the most you can sue for is actual damages, which in most cases is, you guessed it, nothing. You can only sue for punitive if the work was registered before the violation occurred.