Yeah, I hadn't heard about places doing that either. Applied last year and got rejection one day from a site followed by invite the next (saying I had been at the top of the alternate list). It puts you in a very weird position as you feel that they preferred a number of other applicants to you. Or at least that was how I felt. But they may not even remember that come interview time.
I'd do your best not to let something like that bother you. I think most selection committees would be the first to tell you that their pre-interview selection metrics/criteria aren't perfect, and that qualified applicants unfortunately quite often go uninvited.
If they're inviting you for an interview (after waitlisting or even initially "rejecting" your app), they're open to the idea of you heading there for internship; they generally want to meet you just as much as you want to meet them. Just think of it as your opportunity to shine in person.
