They know the line can't protect for a real vertical passing attack. They know the Rams don't have a QB that can truly threaten them even if they do. They know they have to keep receiving options in to help their second overall pick franchise LOT. They know Austin isn't much of a threat running traditional WR intermediate and deep routes.
They saw what we could all see last season and to some extent in the preseason and defended it. Gurley and Austin are running threats, so they defended the line of scrimmage. That's all teams have to stop. I tried to tell people teams would adjust to the way they use Austin and to the only way Austin is actually dangerous. The 49ers did.
Of course, we can't forget the one time Austin did get open down the field he did what? Yeah, that's why teams are going to say if the Rams beat us down the field, fine, but we're not going to let Gurley and Austin as runners beat us.