This means of drag & drop is by default tile-based, not image-based so it doesn't factor in alpha coordinates. It's purpose made to work properly with game objects such as events & the player.
Never much much thought into it before now but theoretically, it shouldn't be too difficult to add a plugin option to swap between left & right-click drag & then have the unchosen option default to event activation. Could probably get it done pretty quickly so I'll look into it.