I also can't figure out why an airline would enforce this..... especially when traveling to Europe.

Let's say I fly to LHR on BA, and then while spending time there decide to head to AMS for the weekend, using another BA ticket? Would they honestly find fault for doing such a thing?

I guess BA would RATHER I book my ticket on another airline such as EasyJet.

CRAZY!!

I wonder what happens if I fly AS to BOS, and then BOS to MAD on AA. And then once I land I decide to take a side trip to ATH.

AS doesn't fly to MAD, AA doesn't fly to ATH. I can't imagine an airline having a problem with this. (Then again, little surpirses me these days.)

Could some give clear examples of what "Nested", "Back-to-Back", and "End-to-End" tickets are?

Thanks all!