That seems very reasonable, doesn't it? Otherwise the fork could pollute the training data through bad modifications, and tracing that would be a pain.
What? It's literally open source, you can ssh into the thing and change whatever you want. I am running a fork of a fork of the code right now. I change things all the time.
You can run stock, or any fork simply by providing the URL of the version you want to run.
Where exactly is the restriction?