The misuse of the rel=”nofollow” attribute can harm the structure of the CC web, and may be even against attribution and non commercial terms. I’ll explain why.
The nofollow, introduced by Google, tells search engine robots to not give weight (relevance) to a link. That, for example, is an effective way to discourage spammers that drop links in blog comments to give an artificial weight to their scammy webs.
But, when a CC author requires that the attribution of a work includes a link back to the source —perhaps, because he wants to ensure that future users can check any clarification or notice about the situation of the work—, adding a traffic and/or semantic restriction to the link means that access to the attribution resource has been arbitrarily blocked by the receiver of the license for certain modes of access, using a method which original purpose was to restrict potentially malicious links. What you must keep in mind is that, actually, the Creative Commons license already contemplates that the link should point only to the page where the author’s attribution and licensing information can be found. That’s it, it must not be malicious. You cannot modify a Public Domain work, license it with CC-BY, and ask for a link back to an unrelated resource.
Therefore, it could be regarded as a violation of the BY terms if you block in any way the access to the attribution and licensing information resources that the author deemed to make accessible. It doesn’t matter that there aren’t actually many tools that will stop the user from navigating a nofollow hyperlink (maybe, if you work at Google and use one of their search engine robots). The fact is that you added an unnecessary restriction.
Besides, when that restriction is used against CC attribution, the network access tools become unable to build a proper web structure to weight the original source of the work above the redistribution agents. Why is it important to weight more the source? Because the source work is the only one managed by its responsible. Simple, huh? For example, if you require support about the work, you are more likely to get if from whoever created it. If later the work is found to be at fail of some license conditions from third parties, or someone sneaked in malicious content, the first place where the rectification will be available will be the source. Actually, if you get the content from a third party and it includes malicious content, you’ll need to find at which step it was introduced to claim responsability: things will get a lot simpler when you get it from the source.
So, any bias that doesn’t weight the source above the redistribution points is likely to create thousands of unnecessary issues and complications to users that could have got it from any of both. That certainly seems uncompatible with what gives sense to attribution.
Now, imagine that you build a non profit webpage with thousands of CC books with “nofollowed” attribution links. Your page is popular and receives lots of “link juice” from people linking it, but that link weight isn’t distributed to the sources of the books. Once it reaches the #1 place at Google Search results, you delete all books, put a price tag on the site and sell it for many thousands to a black hat SEO. You didn’t sell the books, since you deleted them, but the effect of using them is still being carried through by the lasting impact of nofollow. So, you make an economical benefit now by the use you did of the books, thanks to nofollow. You bet someone could find a lawyer who would argue that, if you ever nofollowed the attribution of a Non commmercial CC work, you won’t ever be able to sell the site without breaking the license if your redistribution had any impact on the value of the site.
What I’m saying with all this is that doing tricks with the meaning of a license is tricky. If the license asks for an hyperlink, the most basic form of linking is the safest. That applies too to Creative Commons authors that stretch the purpose of that link, so there’s no reason to be taking “preemptive” measures against them because they “may” break the terms of the license. Sure, you wouldn’t like your government put sexually inhibiting chemicals in your water supply, just in case you become a molester…
