> I do have opinions as to why it wasn't treated as an "advisory" vote but those are just opinions

I'd like to hear them!

Thanks to your comment I re-checked the Labour party manifesto for the 2015 general election and you are right. They do mention a referendum but only in the case of a transfer of power from Britain to the EU:

"Labour will legislate for a lock that guarantees that there can be no transfer of powers from Britain to the European Union without the consent of the British public through an in/out referendum."