Kavu,
I think what we are talking about is confused by syntax.
At our company, which would be the mainline flight you took from ORD to LAX, the cockpit is locked and barred from pushback to arrival. That procedure has been in effect since the late 70's. Because of Crew interaction, (between Cabin and Cockpit), I cannot imagine the door being left unlocked and unbarred.
However, there are several exemptions which allow the crew to come out during taxi and flight, and I think this is what confuses you.
For instance, mandatory by regulations, to ascertain the conditions of the wings pertaining to de-icing before take off.
For physiological needs, bathroom, perhaps walking to prevent deep vein thrombosis.
To deal with certain irregular procedures.
And there are others which could be justified.
There is a protocol to be followed by both the cockpit and cabin crew if the cockpit dooor is open in flight, however , this forum is not the place to post it.
Inclosing, I'd like to point out that the salient point is not the crew coming out.
It is about other people getting into the cockpit.