Nice to see an explanation on surprise. Only took about 10,000 pages of debate here to catch their attention that it needed some treatment.
I can understand their ruling for "no surprise once combat has started" since that is simple.
OTOH, to me it's trivial to add "you can still be surprised during combat, making you unable to act against your surpriser. You can still take actions you would have taken otherwise, actions directed at those foes that haven't surprised you."
If you're fighting goblins, and my Orc surprisingly jumps out to swing an axe in your face in round 3, to me it isn't difficult to tell you that in round 4, you can keep fighting the goblins, but you can't take any action against the Orc, since he surprised you.
Effectively, you lose your action, but against the Orc only.
So if you tell me you attack the Orc, I say no. If you tell me you run away, I ask you "is it because of the Orc?". If you say yes, I tell you you can't. If you say "no it's honestly because there's too many goblins" I might believe you and allow it. This works because you the player and I the DM aren't competing against each other.
If, on the other hand, you say "I keep zapping goblins" that's perfectly okay and not something the Orc can stop. The fact the Orc surprises you does not prevent you from taking that action, since it was something you would do if the Orc weren't there.
Again, writing down a rule to cover this is something I accept the designers don't want to do.