Interesting, I won't pay the taxes this year and I will prove that was unintentionally and see what happens... I am quite sure the outcome is going to be different.
As for yours (which raises a very legitimate question), Vice has an article that mentions that element:
http://www.vice.com/read/why-clinton-isnt-being-charged-by-t...
Basically, the key is that it must be deemed "intentional" or "gross" negligence, and nothing they found rose to that level (in their opinion).
Of course, when mens rea is part of the sentencing criteria, there's lots of room for interpretation, which will ensure there will be a limitless source of fodder for Clinton's opponents and proponents to argue about for years to come...
Other things that you can do are strict liability (or no Mens Rea component). The simplest example is speeding, which can be a criminal charge without a Mens Rea test.
Also note that many civil things don't require a Mens Rea test either, such as tax dodging.
Unfortunately, at this point whether or not you believe in the ruling becomes a matter of faith.