I think this is about more than the signing of the apology letter, it's also about the way that Forstall presented Apple Maps to the world as a perfect finished product even as he reportedly knew that there were serious flaws in the data. That's what made the apology necessary.

Apple Maps should have been presented as a first step in a great solution and the issues acknowledged (with promised improvements) far earlier.

Forstall has been a wonderful engineer and manager at Apple and he is largely responsible for the success of iOS, but letting him avoid taking responsibility for this might've been poisonous to Apple in the long term.