without knowing the case,
the ruling may have simply been,
others had access to it, so it would be hard to convict a single user.
even if the pc was this prof's personal one.
otoh, i find it impossible to believe that a criminal act (child porn) would be sanctioned by any court on the planet.
but let us define child porn - children engaged in sex.
the supreme court ruled long ago (in the case of a coffee table book of nude (?) children) that
child nudity in and of itself is not pornographic.
in that case, the professor may be morally bankrupt, depraved, but not guilty of a crime.
remember, there's lots of legal stuff that is immoral or deadly:
cigarettes, adultery, porn, abortion, sh*t-faced drunk, banking, etc. etc.
but none of it is criminal.