Why are people allowed to play ranked with 0 mastery points?

Just had this 0 point first-timer Xerath in Ranked who thought it was a fun idea to 'try him' His words: 'oh its just bronze, first time xerath, who cares about this elo ranked' He obviously completely ruined the game. Flaming inting the whole package. Why is it allowed to play champions in Ranked you have 0 points with? I think you can prevent it by locking ranked games for champs until you hit lvl 3.
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