Bulgaria Air is not in the one world alliance so the fact that US Air actually rerouted you via London is surprising to start. Iberia does have an agreement with FB but US/American does not.
The only wrongdoing here is the second not-changed ticket -- did they give you Boarding passes or any documentation showing that it was indeed moved via London? The rest is above and beyond service - technically speaking you had a ticket to FRA with US Air and they changed it to London to help you catch another flight...
On a separate matter - this was a tight connection in PHL in the best of days and anything going wrong would have blown it. Considering that you had non-alliance flights after that, I would have not risked it.
What does the travel agent you used to book said when you called them when you missed the connection? Did they tell you to deal with US air or did they give you another option? If you never called them - why? One of the reasons to use agents is exactly this kind of situations.