Because not everybody can afford a high-end ISP that puts them at 15ms every game in the middle of a thunderstorm. Because this is a worldwide game that's not restricted to areas with good Internet. My relatives were restricted to Comcast; they didn't have a choice, and still don't.
Let's say that they wanted to play League (they don't). You're saying that these people, just because of the location they live in, should not be allowed to even play intro bot matches in this game. BECAUSE OF WHERE THEY LIVE.
What about people who can only afford the $15 internet that puts them at 80-120 MS? Now, because of how much money they make, they're not allowed to play this free-to-play game. ** BECAUSE BY YOUR STANDARDS, THEY DON'T MAKE ENOUGH MONEY TO BE ALLOWED TO PLAY IT.**
Think about that before you get all high-and-mighty with your "I can afford the $200 a month 15ms always deal". League isn't a private school where all the rich people go so they don't have to put up with the shabbiness of worse-off people. It's a free game that's distributed almost worldwide.
If you don't want to deal with people who lag, start a Kickstarter campaign for Riot to make you your own god damn private server.