Error Code XQ0058

In "XQuery 1.0: An XML Query Language" / F Summary of Error Conditions (Non-Normative),
it is stated that:

err:XQ0058

    It is a static error if multiple schema imports specify the same target namespace.

I am wondering why is this considered an error, given that XML Schema allows for more than
one schema files to have the same targetNamespace. In fact, it allows for multiple schemas
sharing the same targetNamespace to be combined using the include mechanism.

Thanks,
Panagiotis