What is wrong with single character edits?
It's fairly impossible to make spelling corrections if there's only one error in the post, and unlike the other SE sites, spelling is of paramount importance here.
The reason a single character edit is not allowed on EL&U is not different from the reason it is not allowed on other SE sites. For example, one user could notice that the name of a C# class in a question on Stack Overflow (or Programmers) is misspelled, but users without the privilege of editing posts are not allowed to make that single change.
It is true, the spelling on EL&U is important, but consider that:
The other reason single character edits are not allowed is probably to avoid a user gets reputation for just changing a single character.