MAC addresses specify the backoff time for collisions on a LAN and aren't used at "worldwide" scales. They get stripped by the first router that sees them.
MAC addresses have nothing to do with collision back-off time. The back-off time is an algorithmically specified value that depends on the number of collisions (up to 16) that have occurred while attempting to transmit a frame and a random number. Collisions only occur on half-duplex Ethernet which is not normally used on modern LAN implementations. They are dropped (along with the entire frame header and CRC) by the first layer 3 (routing) device to process the frame, but are potentially used globally when an Ethernet frame is wrapped in another frame such as in some VLANs and increasingly popular Software Defined Networks (SDNs). Since they are guaranteed globally unique, they can be very handy for many things.
Only hardware vendors that need to provide unique collision avoidance characteristics on any customer's LAN need MAC address allocations.
Again, MAC addresses have noting to do with collisions and most LANs installed in the past decade have no collisions. They are used for addressing at layer 2 of the OSI reference model (Data-link). They are not actually a part of the Ethernet (IEEE 802.3) spec, but of a more global specification for creating globally unique hardware identifiers for network devices (IEEE 802) and are used by several LAN types which never had collisions (E.g. token ring, MAP (EtherBus), and FDDI).
As has already been pointed out, 16 million is the SMALLEST block of MAC addresses assignable, so this is far, far from massive. It does indicate that they plan on providing a globally unique ID for every machine which may or may not be actually used for addressing purposes.