Android Architecture Components network threads

I'm currently checking out the following guide: https://developer.android.com/topic/libraries/architecture/guide.html

The networkBoundResource class:

// ResultType: Type for the Resource data
// RequestType: Type for the API response
public abstract class NetworkBoundResource<ResultType, RequestType> {
    // Called to save the result of the API response into the database
    @WorkerThread
    protected abstract void saveCallResult(@NonNull RequestType item);

    // Called with the data in the database to decide whether it should be
    // fetched from the network.
    @MainThread
    protected abstract boolean shouldFetch(@Nullable ResultType data);

    // Called to get the cached data from the database
    @NonNull @MainThread
    protected abstract LiveData<ResultType> loadFromDb();

    // Called to create the API call.
    @NonNull @MainThread
    protected abstract LiveData<ApiResponse<RequestType>> createCall();

    // Called when the fetch fails. The child class may want to reset components
    // like rate limiter.
    @MainThread
    protected void onFetchFailed() {
    }

    // returns a LiveData that represents the resource
    public final LiveData<Resource<ResultType>> getAsLiveData() {
        return result;
    }
}

I'm a bit confused here about the use of threads.
Why is @MainThread applied here for networkIO?
Also, for saving into the db, @WorkerThread is applied, whereas @MainThread for retrieving results.

Is it bad practise to use a worker thread by default for NetworkIO and local db interaction?

I'm also checking out the following demo (GithubBrowserSample): https://github.com/googlesamples/android-architecture-components
This confuses me from a threading point of view.
The demo uses executors framework, and defines a fixed pool with 3 threads for networkIO, however in the demo only a worker task is defined for one call, i.e. the FetchNextSearchPageTask. All other network requests seem to be executed on the main thread.

Can someone clarify the rationale?