This disease is caused by a specific bacterium that is found in freshwater sources. It typically grows in war water environments. For it to become a real problem, water droplets with this bacterium must enter the lungs. Most disinfection systems are going to take care of this problem before it ever enters drinking water. This includes before the tap and after the drain.
To be honest, you’re more likely to catch Legionnaires’ if you use a public hot tub more than drinking your tap water. If you are concerned, I’d talk with your local water and wastewater authority to ask them about how they control bacteria from your drinking water sources.