Ah fair use is a defense in court, not something you can trot out in all situations before you get to court.
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.
Fair use is as much a legal right as copyright protection itself; it's codified in 17 U.S.C. § 107. You don't have to argue before a judge before you can use a work without permission under the fair use doctrine -- thankfully.
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".
What I'm trying to get at is that every provision concerning fair use is murky, and has to withstand argument in court. Every part of a fair use claim is subject to 'judgement' rather than merely establishing factual claims.
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.
>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.
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.
