It's interesting, but this seems to be an exercise in elaborate stupidity that is bound to backfire horribly.
For this thing to work the DNA squirt cannons have to be activated during or after the robbery. This is a very complicated version of numerous active security systems that already exist. Things like triggerable cameras, paint based marking systems, and other assorted things. The idea of putting something in doorways or in a parking lot to activate from inside to nab thieves is old and has taken plenty of forms over the years.
The thing is that this encourages more violent crime in the long run. Now, someone wanting to rob a Mcdonalds is likely to be nastier about it and shoot (or bludgeon to death) the workers figuring they are going to be caught anyway, or tie everyone up. Basically your
going to get more hardcore criminals and crimes.
This might have worked for a little while if they didn't tell anyone about it, but now with it known, it also means robbers are going to plan ahead and be nastier. It's not that difficult to circumvent... and we're already dealing with armed robbers by definition here.
This will sound odd, but I'll be blunt, the best way to deal with problems like armed robbery are to keep very little cash on hand. If there is a rash of robberies of Mcdonalds in Australia then the company should invest in getting security runners to pick up the excess cash a few times a day and take it to the bank, keeping the minimum needed on hand at any given time. Hiring a security company to do that is probably going to be cheaper and more effective than buying, installing, and maintaining bloody DNA squirt guns.
See, those after money aren't going to go after a place they know won't have much cash, and if they do it's easier to just let them have it and take a minimal loss than to try and turn your business into Fort Knox.
I also await the inevitable lawsuit when one of these dispensers leaks and someone claims wierd side effects from the DNA. Not to mention urban legends involving this stuff leaking into the burgers.