That is not an example of a double standard.
Reproducing code-only bugs is not bannable unless you exploit them to gain a competitive advantage. Emphasis on exploit and competitive advantage. Discovering them by accident in a PVP is not against the rules. Trying to figure out the reproduction steps in a non-competitive environment is also not against the rules (given the Bug Reports forum specifically asks this of its users).
As for those cases: as you say, some exploits do require injection. Altering the game's code is a separate bannable offense. If those people altered the game-code then of course they would be banned, even in custom games.