"I was assuming the parent poster wasn't so much a terrorist as a mischievous prankster"
How do you tell the difference? A dirty bomb is mostly a weapon of mass distraction. The response is likely what shuts down an important area, rather than the actual danger.
Doing it with a sizable number of relatively harmless sources spread out over a block or two will keep them guessing what the danger and scope is for a bit, even if each one isn't particularly dangerous. It doesn't have the extended clean up phase, but they still have to evacuate, check people as they exit, and then determine what the devil is going on. It also gets a lot of attention.
The individuals who placed them can just claim it was a prank that got out of hand. You'll probably still get jailed, but it'd be hard to justify a life term for it.
At the same time, AQAP, for example issues a claim of responsibility. No one is really sure what the straight of it is. More confusion, disruption and doubt.
Maybe they can still link those who placed the sources to a higher level group, but it's still a lot easier to recruit pranksters than hard core murderers.
You can also use several of this sort of incident to get people to stop reacting to it (alert fatigue), and then release something that initially looks similar to the detectors but is really much more dangerous.