Global Admin should have capability to view Reserved Instance reservations.
Global admins SHOULD be able to view (or at least manage permissions) of Reserved Instance reservations in the Azure portal. Currently, only the purchaser and account owner can manage the permissions. This is contradictory to everything else in Azure, where a Global Admin alywas has the capability to change permissions (acting as a User Access Administrator).
On a secondary note, Read-only admins in the Enterprise Agreement portal should also have the ability to view reserved instance data.
Lester W commented
By the way, you can view and manage via the API just fine. It appears go be purely the Azure portal that is stopping access.