It seems like its an important thing to be able to express. They are extending their trust to the new CA. I believe this information is available to clients if they wanted to choose to reject cross signing.

You're right that this sort of broad power is scary, but I think it's being used reasonably in this context. Do you have specific concerns you are afraid of? The only problem I thought of was a compromised CA cross-signing a malicious CA, but if they are compromised, you could just issue the main CAs certs anyway.