The attack on Melania Trump is akin to the attacks on President Obama during his candidacy in 2008. Such attacks, regardless of the political spectrum one falls on, illustrate the negative, ugly side of U.S. politics.
People attack one another in hopes of at most disqualifying them, and at the least, tarnishing their image. In this case, it is safe to say that Donald Trump has said and done things that should in theory hurt his image, yet they push they propel it. Therefore, his family, primarily his wife, and her at times questionable past, become the victims.
It is morally objectionable, yet it is part of the process. The United States system is known for its transparency ( more or less). In this case, the actions against candidates and their families, which can be considered harassment to a certain degree, are just the effects of transparency, and the right of public knowledge–especially if the person is going to become the next POTUS or FLOTUS.
They in someways safeguard against ineligible candidates, but it is at the cost of personal privacy.