The work performed by European scientists was probably mainly funded by European taxpayers, and the same argument that American taxpayers should be able to access the work they fund can be applied to Europeans too. As an European taxpayer (and scientist, BTW), I think freeing this work from the paywalls is morally great, even though it's illegal. And paywalled academic journals should adapt or die.
There is no parallel between "works funded by American taxpayers" and "works funded by EU taxpayers". The un-copyright of works funded by American taxpayers comes from a specific exception in USA laws - there is no such statement in European legislation; so while ethically the situations may seem the same, legally they are very different.
