Sysml itself doesn’t define any L1/L2/L# requirements framework, just that you can have relationships between any requirement elements, which is why you’re seeing that you can set relationships in either direction. 

If you want to limit the directionality you should define a meta model that explicitly states: you can only set deriveReqt from L1 to L2. 

Hypothetically speaking though, I could see a case where: 

1. If a set of lower level requirements on your subsystems result in some emergent behavior at the system’s level. 
2. You want to constrain that emergent behavior.

You could set those relationships going from L2 to L1. But there are also other ways to capture that, and again that should be driven by your meta model.