Or you are planning an extended terror campaign across the US using cell phone triggered IMDs. They don't need this many phones for day to day communications.
They do if they are buying for the whole terror network. And if they expect the US to crack down on the sales (stockpiling).
This is dual-use technology. Make detonators from a few, use the rest to communicate (imagine how many phones you would use if you had to throw away each one after 1 call). Thanks to the NYT, the terrorists now know what they have to do to stay hidden.
There is the possibility that these are just Muslim kids buying phones on the orders of the local imam, without knowing what they were doing. They should still go to jail for it (ask questions next time...).