Ignite says true damage! Then why does a shield take the damage for you?

I think that it is so ridiculous that ignite clearly states that it does true damage based on lvl. Then why in the heck does a janna shield or a karma shield or any champ that shields for physical damage soak up the damage of the ignite for them. in my mind true damage means direct damage to the players current life. penetrating any armor or magic resist or shield that would be put on the champion ignited. riot should clearly either reword what ignite is actually doing by stating that it does physical damage based on lvl if there not going to fix the true damage problem or fix the true damage problem and make it penetrate any shield, armor, and magic resist on a player

Thanks for reading Demaciasblight