There are different dummy cartridges, some have dimpled or fired primers, some have a rubber insert for primer, some have holes drilled in them. Some have no way to distinguish them from live ammos - the pic above is for an ad for dummy 45LC. The only saving grace of those rounds above is they are sized so they can't be loaded into a 45 cal gun.
See: http://www.historicreplicaguns.com/brass-dummy-cartridge.html
It is true that you can make them any way that you want to. I consider the commercially made dummy cartridges to be a complete waste of money. They charge a lot for something that can be made basically for nothing. Personally I prefer to use an already fired primer in mine, and I mark up the cases with a permanent marker. The ones that I have made have mostly been used for training purpose to show people how to load handguns, rifles, shotguns and magazines.
So far I haven't heard anything from a news source that indicated that dummy cartridges were being used although this would be the most reasonable explanation for what happened.