Because there isn't a "good chance they'll resist with Edge anyway".
In your mind. Not necessarily the mind of the hypothetical player in question.
Inaccurate. Let's look at the simplest way to do this - the first time a player summons a spirit, it's "Edge on oversummoning, and oversummoning only"; if he has Magic 6 there is no chance it will resist with Edge. The player's actions may
transparently influence that mark in either direction - the sorts of things that are considered to be abusive or disrespectful would need to be outlined off the top, as would the reverse. So, no, the player cannot in fact reasonably think that because he's positing something that he knows to be false.
Michael: Would you disagree with the idea that if abuse and disrespect can move the "resist-with-Edge" barrier to lower Force, then respectful and kind treatment should do the reverse?
With how subjective the "mistreating one's spirits" is, it should not only not be all that common but also come with warnings--to the player, not the character--that certain actions may precipitate such starting as they're brought up, because what the GM considers "mistreating them" may be well before the proverbial line to the player.
And that's why you outline it off the top. There has to be a mutual understanding of what will be considered to be negative or positive. How common it is is completely up to the player - a player frequently using spirits as meat shields doesn't make using spirits as meat shields fine and dandy.