You're absolutely correct: it's perfectly legal to download free articles. But this is why I made my "semantics matter" argument a few days ago: He said "download and free," free being a verb and intending to mean "set them free."
Yes, there appears to be an allusion to Aaron's particular case. However, one could certainly see where a particular journal publisher holds the copyright on many excellent articles made available online for a fee having to protect their copyright from some unscrupulous academician using their paid acces to liberate those writings. And how, given the predilection of prosecutors to scare would-be defendants, automating that task might see the threat of criminal charges.
To actually respond to your comment, it may be legal to download free articles (and it's probably legal to download articles for which you've paid), but is it legal to distribute someone else's copyrighted work without their permission? Price doesn't matter.
Like with the Swartz case, learning the facts involved sways our opinion. Learn the facts on HSBC:
https://www.youtube.com/watch?v=I2Sr2JbC4WE
(by Young Turks host Cenk Uygar)
Edit: no offense. Not sure if I grok your comment or you grok'd GPs.
Only penalty was a month's worth of profit. Not one person will serve a day in prison.
But hey, don't unlock your phone, it's a felony. Or download and free journal articles.