Because the jurors are not supposed to associate with the involved parties during the trial, with the exception of what is necessary in court. And had the defendant not reported it, she could have been punished as well. So at the very least, the answer is to avoid punishment if the judge and/or plaintiff/prosecution found out about it. As for the justification of the law, if a juror is friends with the defendant on Facebook, that could bias him or her towards said defendant. Which is not a fair trial. So if there were no law about juror-party communication, perhaps the defendant would have been wise to keep her mouth shut. But she would be legally obligated to report something like that, as it is considered tampering, and rightly so. Did the juror deserve to go to jail for it? Probably not, although he should definitely have been kicked off the jury and replaced. And probably fined too. But that's besides the point.