Actually, they did. Maybe not blow it up, but at least disable it. The suspicious part isn't so much that the cellphones were taped to things, but that there were so many of them. Lithium batteries make a very nice explosion and all you need is something to short them with.
Hmm, I guess the tape could have the same effect as it does on sparkler bombs. No tape=sparks. Taped up=BOOM.
And if you try this at home, careful, there is sometimes a second explosion if the cells go separately.