Why do Pineapple and Kiwi Ruin Gelatin?

As far as I'm concerned, the video could just as easily be asking why thumbtacks or dog poo ruin gelatin because I'm about as likely to put them in my gelatin as I am to put pineapple or kiwi.

...because those fruits are nasty.

Sure, these acidic fruits denature the proteins that hold together gelatin meaning that they ruin the collagen network that keep the gelatin jiggly. That's what acids do, donchaknow.

I assume that highly basic foods would do that some thing, too.