Why does a jailbroken iPhone always ask to restart Springboard after installing a new app?
It doesn't always - only when the new app directly affects the interface. So when you install command-line only functionality there is no need to respring - at the other end of the scale you would also need to respring every time you change a theme in, say, winterboard, even though you are not installing a new app.  

The reason for this behaviour at both ends of this spectrum is that none of the jailbreak 'tweaks' can affect the springboard while it's running - they have to change the filed copy and then restart with a new version of springboard...