(I say this as an author of one of those dead-tree things, and of much software. I'd like people to follow my license agreement, but some in some cases copyright law grants my users more rights than the license, and I respect that.)
Also, I doubt that all of Pinterest users' use cases are protected. Depending on the proportion, it might be more apt to say that these content owners are proactively protecting their content from unprotected uses, rather than resorting to the courts or DMCA.
Edit: wow, this is an incredibly contentious topic. Never seen this density of downvotes in a thread before.
Whilst that might be accurate in the detail of the law, it appears to contain something basically malfunctional.
It seems to render the law effectively meaningless. If you are actually prevented from doing something, what has happened to your right to be allowed to? If there were public rights-of-way across someone's farmland, but they built walls around it, there would seem to be some kind of problem. (Or maybe a better example would be a visibility-cloaking device that made it look like there was a wall -- is that prevention, or just making things less easy? Does that make it OK?)
If the letter of the law supports the frustration of the intent of the law, something is wrong, surely.
There's also a difference between a positive right (the government protects your right to do something) and a negative one (the government will not prevent you from doing something). Fair use seems to be a negative right -- the government does not prevent you from using copyrighted works. But it doesn't require that copyright owners convey these works to you as conveniently as you want.
Secondly, consider the context... how are you going to argue that people have a fair use right to share copyrighted images with the public simply because they want to show what they 'like' on their pinboard?
Allowing that makes a loophole big enough to render pretty much any copyright protection of images null.
A good portion of US law is the "meet these conditions or this protection doesn't apply" type; fair use is no different. We don't go around telling people "don't run user-generated content sites, the Safe Harbor provision of the DMCA is just a defense in court".
The law doesn't provide any real guidance as what constitutes a fair use work, only that somethings do count as fair use and in court we'll decide which do.
Determining fair use is a complicated, fact-specific analysis and even lawyers will have a hard time predicting if a case will win under a fair use claim.
Lots of things are items 'to be decided in court', but the preponderance of prior cases, and the strictness of the law makes it pretty clear if an argument is going to be successful or unsuccessful. Fair Use doesn't really provide that.
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Edit: I started this discussion on the wrong foot... we're talking about how the laws apply globally, but my intention was to remark about Pinterest and Pinterest users only.
You can say the same thing about copyright law - both are provided for and are explicitly provided, and both are a matter of judgement.
Here's the thing: As provided in the law, fair use and copyright are on equal footing. One does not carry more weight than the other. Both "rights" exist. You can copyright something, but that does not strip other parties of their fair use rights.
I guess you disagree.