Those mixture screw restrictors limit the amount of turning the operator can adjust. In other words they shouldn't be so far out of adjustment that it won't run, but that's the theory. Most of the time I pull the plastic caps off so I can adjust them properly after cleaning the carb well. Personally I HATE em but put them back on the weed eater or what-have-you if it belongs to a customer. More often than not I file down the limiter tang and then can turn it anyway I want and still have it on there!
