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The system is a bit harsher at lower levels for a few reasons:
1) it's based at least partly on number of games played. Basically, 1 leave out of 6 or 7 games means you've left roughly 15% of your games. If it's 1 leave out of 50 games, that's 2%.
2) They want new players to understand that it's not acceptable to leave games
3) It helps weed out trolls who don't care that leaving wrecks the game for the other players
Basically, if you knowingly play with an inconsistent internet connection, then yes, it is your fault. However, surprises happen and as long as you don't make a habit of leaving, your leaver level will drop and your won't get pinged again.
BTW, I left on the first game I ever attempted and only got a warning. That was about two years and several hundred games ago and I've never had more than that one warning.