All of the index methods have their own synonyms of the BUFFER_LOCK_* constants, for example:
#define GIN_SHARE BUFFER_LOCK_SHARE
#define GIST_SHARE BUFFER_LOCK_SHARE
#define HASH_READ BUFFER_LOCK_SHARE
#define BT_READ BUFFER_LOCK_SHARE
But most of them pass their constants directly to LockBuffer. So if they were ever defined to be anything else, things would fall apart pretty comprehensively. (Hash index also passes them to LockBuffer, but only indirectly via some utility functions).
What does this pseudo-encapsulation get us? It seems like we have a separation of spelling, but no real separation of concerns.
Cheers,
Jeff