I'm not sure if adding PPPOE will be on the EX roadmap or not. You can check with your sales engineer or partner.
I would guess that Juniper sees this feature as one that needs to have the security package features on the device to use. Since you would generally be connecting to the internet with such a protocol they would expect it would be terminated on a device with security policy enabled. Hence the SRX, J series and ScreenOS implementations.