When you remove a branch from route or split the presets in that branch are also removed. For this reason, presets that alter the structure of the workflow (route and split) are not shared between workflows.
When you save those presets as a Global Preset you are in fact saving a template that's used to create unique presets within a given workflow. Unique presets must have unique names as well.
Route and Split presets are always unique to the workflow for the reasons mentioned in the thread above. However, they are still separated from the workflow – as are all other nodes. For this reason, they must all have unique names.