Save the Date: Recipient Response is ??????

I'm confused (but it does not take much).  I received a 'save the date' email recently for a wedding.  The email contained the date, time, and location of the wedding.  It also listed recommended hotel accommodations for out-of-town guests.  There was an email to contact the couple, along with adorable pictures of them. 

Am I supposed to respond to a 'save the date' announcement?  Is this an actual invitation?  I guess why I'm asking is that a friend of mine got a 'save the date' about a year ago from somebody who was getting married.  As the time of the wedding got closer the couple decided to ditch plans for a huge wedding and opted for a small beach wedding. 

The would-be-guests were PISSED.  I guess I can see why.  Jilted guests had put aside sacred vacation time months prior that could not be altered.  They also endured making travel arrangements, bought tickets, and planned their summer around an event where they did not make the "A" final-cut-guest-list due to a reality check in the bridegroom's wedding budget.

My friend was not jilted as she's a local and did make the final A-list cut.  The wedding couple justified their actions by stating that the save-the-date "was an announcement, not an invitation."  hmmmmm.... help me out here...
So, is the "save the date" an invitation, or a preview of coming attractions?  How does the recipient tell the difference?  What does the recipient do when an actual invitation does not follow as the save-the-date grows closer?  Does one call the couple and meekly inquire if your invitation got lost in the mail?  How does the recipient tactfully do that?  What if the couple decided not to go through with getting married (which happens frequently)?   All of these 'save the date' points are worth pondering.