yeah that's a bit weird...you can also click on the automation line to create a new start and ending point ("j" locks it to grid or quant, whatever you've set), then you'd need another set of points within those start and end points, select the inner 2 (just click somewhere and frame them), and drag them down both together.
Or in the object bar (if you've turned it on), you can drag the value up/down, or use the mousewheel for that.
You have to be careful to not write new values when creating the points that way though, I'm sure theres a shortcut to lock them, but I don't know it right now, haven't been mixing in Cubase for quite a while now...