I've got to say - I found it REALLY difficult to find an authoritative or trustworthy source on what the link tax is.
From my understanding here's the rub: there are two definitions. One definition is you'd have to pay a publisher if you link to their article whilst quoting it. That's what the opponents are saying. This seems disingenuous, since it's the quoting the article that triggers the tax - no the link.
The supporters characterize it as: If you rip off a significant part of a copyright work the publisher can charge you since you're taking their copyrighted work (whether you link to them or not).
Now people are saying sites link Google News would be affected because they list lots of news sites quotes with links to the source. I would've thought it was pretty obvious however, if the aggregater quotes enough then there's no point to click through and therefore the revenue accrues to Google rather than the publisher. This seems fine to me -- there should be a charge for that behaviour. What's more it actually seems self-levelling. If Google doesn't quote too much and people click through then there's no incentive for the publisher to charge Google for the content since they're generating demand and charging Google would cause them to be de-listed.
So let's move on, why is Google complaining? Well it seems to me that it's actually in Google's interests to be able to leech off these publishers by stealing their content wholesale and this would prevent that, and apparently they think whining publicly is a reasonable strategy. It's not.