Can someone explain me why there are bots in normals?

I was trying to get Mastery 7 on darius and akalo for nearly 1 years! but today I found people with ghost and heal who came and basic attack me with 10% hp! And by killing them all, I got my mastery 7 on both akalo and Darius! first I thought they are just random players who think they can outplay me with 10% hp under my turret... but then I came across a board thread about bots with a ghost and heal! I have two questions : Is getting S by killing bots considered an exploit and is bannable? Why there are bots? who put them in normals? rito or people who are boosting? ty for answering!
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