Question about mouse-up and mouse-down to emulate keyboard keys.

One of the really useful things to do with extra mouse buttons is to assign them as modifier keys. On my favorite mouse I have three thumb buttons. I use BTT to assign one as command, the second as control, and the third as shift. This allows me to keep one hand on the mouse to hold down the modifier key while using the other hand to hit some keyboard key, no taking my hand off the mouse for the modifier key or using one hand spread across the keyboard to simultaneously press a modifier and some other key. Copy and paste are very common uses for this but a huge number of useful commands can be performed in many applications.

The odd thing is the way this must be set up in BTT. I have to set up two actions for each keyboard modifier key that I want to emulate with a mouse button press, one for key-down and the other for key-up. Is there a reason it has to be done this way rather than just assigning a single action with the mouse button set up as the modifier key? Once I figured out how to do this it became easy to repeat as needed but it was not intuitive and I don’t understand why it’s necessary to set this up as a pair of actions in BTT.