I'm thinking she poured concrete on both sides and covered the sides of the cakes while positioned on the base? But that would mean they had to transport the cakes like that and they could easily fall off the top. I don't see any washers or bolts on the bottom of the cake on the left, though. Humph. Any ideas?

https://s-media-cache-ak0.pinimg.com/736x/f8/8c/76/f88c7654576edc40f2b804943df350c9.jpg

I think that's just the cake stand, so she probably put the cakes on the stand when it was set up at the venue. I doubt that the cakes are attached to the bars themselves, they're probably just sitting on the stands.

 That's awesome!  It looks like a clear disc/plate on top of the pipe.  Perhaps some type of adhesive or epoxy?  It almost looks like there could be a bolt on the one on the left...just lined up with the pipe, but I don't see that on the middle one.