I have posted on here before that I have done it. (Well, I was present in the car when friends did it. In fact, the person driving at the time works in a fast food outlet, as did one of my other friends present).
I have seen ones on youtube which I disagree with, but the general premise that you grab an ice cream upside down is completely harmless to the drive through person. Like thevictor390 said, most of the time it makes their shift. Hey, the cone-er is the one getting sticky hands and ruining their ice cream. They've paid for it, they can do whatever they bloody well please with it. If by some accident the worker gets ice cream on their hands, it's a lot easier to wash your hands in a McDonalds than it is in a car.
I fail to see how 'making a mess of ice cream' would ruin a drive through worker's day. It's not exactly sacrilege.
Besides, it's an old fad now. It's something you do once with your mates, have a laugh, and forget about. I don't think the drive through worker is exactly having long term trauma caused by the uncustomary holding of an ice cream either.
Anyway, other things that annoy me are faulty anti-theft alarms in shops. Yes, I closed the lid of a MacBook air to look at it. No, I am not stealing it. Your sirens are irritating. And it happened three more times to other people while I was still in the shop.