- Add a term for a certain taxonomy with the name: Test
- Add another term for a different taxonomy with the same name: Test
- In the wp_terms table you find one record for both terms. In the wp_term_taxonomy table you find two records with the taxonomy and description and other information about the terms.
- Now, if you edit one term and change the name from Test to Test2 then both terms are changed...
Why is WordPress working that way?
EDIT: The real question is not why WP uses a unique slug for a taxonomy. Thats obvious. It's why WP uses one record in wp_terms for the same term. But the meaning of 'Test' can be different in different taxonomies. If you edit one of the terms to be different. That isn't possible anymore. WordPress will edit both because they are both linked to the same term_id.