What else would they be for? Turnouts are specifically designated in the vehicle code for slower traffic to pull off and allow passing. In fact, I believe 21656VC may be the only the only place in the code they're mentioned.
If it blocks a vehicle from making use of the turnout, one could say that it is. Unlike your hypothetical field, the turnout is part of the highway by official definition.Legal stopping and parking areas, which can be turnouts, are also part of the highway. Since turnouts do not have lanes or marked parking stalls, and a car in a turnout is out of lanes of through traffic, how can he be obstructing traffic? It would be like stopping in an open field and being charged with blocking traffic.
Still it's a rather subjective stretch to claim that an attended car in an otherwise empty turnout that is well clear of traffic either on the main roadway or trying to exit the main roadway to enter the turn out is impeding traffic.

