As we know, street parking carries along its own unique set of risks. Break-ins and thefts are not uncommon, as is vandalism like dragging keys along the paint. Now, imagine the type of miscreant who would commit such an act when he's faced with a fun new target: several blocks and dozens of cars plugged into curbside chargers. Such an individual would likely be inclined to simply unplug all the cars as they walk by. Or worse.
You can put a locking fuel cap on an conventionally-powered prevent gas from being siphoned out, but the proposed locking mechanisms on some charging couplers don't totally eliminate the possibility of someone simply disconnecting the cord. We're not sure what the solution is, but if this sort of thing were to happen on a wide scale, it's just another potential impediment to getting drivers to consider plug-in vehicles.