Attribute mapping is an application specific bit of functionality.
The SAML specification(s) details with things like message exchanges and XML schemas, not the functionality software should provide or how bi-lateral arrangements between IdPs and SPs should be organised. They have nothing to do with the SAML specification. Sorry.
Note that there are plenty of other SAML products that provide similar attribute mapping functionality, it's not just shibboleth that does so. I imagine the problem here is that the Service providers consider their requirements more important than yours and aren't prepared to make an exception. Either that or they don't know how.