Same happens for me but with setting a ROI.

The very first time executing the following code works:

annotationLogic = slicer.modules.annotations.logic()
selectionNode = slicer.mrmlScene.GetNthNodeByClass(0, "vtkMRMLSelectionNode")
selectionNode.SetReferenceActivePlaceNodeClassName("vtkMRMLAnnotationROINode")
annotationLogic.StartPlaceMode(False)

After deactivating programmatically place mode with

annotationLogic.StopPlaceMode()

and then

annotationLogic.StartPlaceMode(False)

the mode is not active

After freshly starting, please run the following code. It's supposed to start the place mode and listen to the event of the interactionModeChange.

def onInteractionModeChanged(caller, event):
interactionMode = interactionNode.GetCurrentInteractionMode()
if interactionMode == interactionNode.Place:
print "started"
elif interactionMode == interactionNode.ViewTransform:
print "end"

annotationsLogic = slicer.modules.annotations.logic()
appLogic = annotationsLogic.GetMRMLApplicationLogic()
interactionNode = appLogic.GetInteractionNode()
selectionNode = appLogic.GetSelectionNode()

tag = interactionNode.AddObserver(interactionNode.InteractionModeChangedEvent, onInteractionModeChanged)

Turn on

interactionNode.SetCurrentInteractionMode(interactionNode.Place)

The output of that will be:
started
end

The following code works as a workaround:

def onInteractionModeChanged(caller, event):
interactionMode = interactionNode.GetCurrentInteractionMode()
if interactionMode == interactionNode.Place:
print "started"
elif interactionMode == interactionNode.ViewTransform:
print "end"

annotationsLogic = slicer.modules.annotations.logic()
markupsLogic = slicer.modules.markups.logic() # needed for setting active list
appLogic = annotationsLogic.GetMRMLApplicationLogic()
interactionNode = appLogic.GetInteractionNode()
selectionNode = appLogic.GetSelectionNode()

tag = interactionNode.AddObserver(interactionNode.InteractionModeChangedEvent, onInteractionModeChanged)

when no list is active in markupsLogic, it won't work

markupsLogic.AddNewFiducialNode()

Turn on

interactionNode.SetCurrentInteractionMode(interactionNode.Place)