Ok. Let's imagine a situation where you're playing in a tournament and contending for the win. Alex the Great signed up and beat you in the first round, then withdrew. Why on earth should the remaining 8 participants get free wins against a strong player whom they could lose to? Is that fair? That's exactly why the 50% games threshold was introduced.
If a person has played at least 1 game, they have already started in the tournament, you can't turn a blind eye to it and say it never happened.
And now let's imagine that I beat Alex, and then he withdrew. Maybe that was the best game of my life, and some "uncertain rule" is stealing that achievement from me. The rule "protects" the interests of the losers from the fact that a person might beat them and then withdraw. It's more fair to "protect" the interests of those who won, I think.