I don't see why you'd actually need any SIMs in the first place if you wanted to DoS a cell tower. My guess is that it's basically just a device farm for either sending spam or receiving activation codes for spam accounts elsewhere. By putting them in a populated area, the increase in traffic is less noticeable.

It makes it much harder to nail down exactly where the farm is. You can't just go break down all the doors in a large high-rise and the reflections of the radio signals in the urban canyon will further hamper your ability to pin-point the devices. But you might be able to correlate power consumption or heat signature with activity.