Hello Michael,
in first PPPoE implementations the PPPoE client was running on user PC and the ADSL modem was acting as a bridge bridging Ethernet PPPoE frames over an ATM PVC over ADSL.
With a modern router you can:
use the router as a PPPoE client on its WAN interface.
Use the WAN PPPoE enabled interface to route to the internet (the dialer interface is actually used as the L3 outgoing interface)
You can use NAT to provide internet access to internal devices.
In other words the PPPoE sessions is an emulated point to point connection over ethernet, only one source MAC address can be used over it in outbound that is the one of the PPPoE client.
For this reason a routing setup like the one I have described above works.
Hope to help
Giuseppe