The reason it's likely not allowing this is because a Bin is essentially a dimension, not a measure..
if i was in the predicament, I might try the following however, I doubt it would work..
Right click the blank area within your columns shelf and type the number '0' then hit enter. This will create a new green pill on your columns shelf with the sum of 0. Make this a dual axes and synchronize it with 'Total System Activities'. Then, depending on the type of reference line, if you'd like to create the reference line based on a measure, put the measure into your marks card of SUM(0) to make it available in the reference line dialogue.
Thanks for the suggestion. And you are correct...it doesn't work the way as expected. I wish there was a way to add a reference line from the x-axis along with using the bin factor as a dimension. I'm not sure how to convert a bin dimension to a measure in order to add the vertical reference line.