In reading the above link, it's obvious that her arrest stems from more than just that one email, but rather a series of email messages and online comments that some had claimed were harassment.
That being said, it most likely is still protected free speech, as odious as those messages and postings were. I think the case will be dismissed, but her future in law school is probably over.
You are absolutely correct that her arrest relates to a number of actions on her part, apart from the email that the quote comes from.
However, without having seen the arrest report or other documents, it seems from the news articles that the quote in my post is considered by the police to be in itself harassment.
Also, to be arrested for protected speech is bad enough, even if the charges are dismissed. Imagine if every time you held up a banner you could get arrested for harassement, it took a court case to acquit you.
That being said, it most likely is still protected free speech, as odious as those messages and postings were. I think the case will be dismissed, but her future in law school is probably over.