sage has a GPL V3 license. How is Jupyter notebook able to invoke it without forcing Jupyter to also be GPL V3? Is it because sage is used only as a binary?
By communicating over the Jupyter Messaging Protocol, a Jupyter client can talk to software in any licensing regime, open source or proprietary, and indeed, a number of proprietary kernels do exist.
As to licensing: in the GPL family, usually only AGPL-3.0 expands the definition of “derived system” to include network-connected components. Sometimes terms of service, trademark and other things come into play (not just copyright), as well.