One important difference in this case is that while "Pylance leverages Microsoft's open-source static type checking tool, Pyright" [1], Pylance itself is not open source. In fact, the license [2] restricts you to "use [...] the software only with [...] Microsoft products and services", which means that you are not allowed to use it with a non-Microsoft open source fork of VS Code, for example.
The license terms also say that by accepting the license, you agree that "The software may collect information about you and your use of the software, and send that to Microsoft" and that "You may opt-out of many of these scenarios, but not all".
[1] https://github.com/microsoft/pylance-release
[2] https://marketplace.visualstudio.com/items/ms-python.vscode-...
If that's not working for you you should open an issue at https://github.com/microsoft/pylance-release.
An example is the Python language server
https://github.com/microsoft/pylance-release
They base it 90% on the open-source pyright library. Then the lock down all their own enhancements.
https://github.com/microsoft/pyright
This is why MIT isn't always the correct choice. GPL wouldn't hurt devs at all and would protect from this kind of garbage from MS.
The server is open source, right? It's here: https://github.com/microsoft/pylance-release. I don't know what you're referring to. Can you link to information about the proprietary extensions you're referring to?
I wonder if Guido will be ok with that. For me pylance looks like the biggest evidence that embrace-extend-extingish mindset is still alive in Microsoft, unfortunately.
https://github.com/Microsoft/vscode-python https://github.com/microsoft/pylance-release