I believe ESL considers it an exploit because it's not only an odd, unintended mechanic that requires multiple steps, but because it breaks the game logic. It's not very skill-based or feasible like jump-puzzling (without a deployable shield), which is a mild, unintended mechanic that is allowed in the ESL. It just doesn't make sense that your character can't breach a doorway they physically appear to be perfectly able to but aren't because of the invisible, arbitrary barrier beside the shield. Whereas a character could feasibly skirt on the edge of a building in a jump-puzzle or Bandit could somewhat believably electrify the metal reinforcements in time to destroy breaching charges on the other side of the wall with his titular trick.
The only difference between using Bandit's batteries normally and tricking them is timing. There's a big difference between using a shield as a shield and placing it in a specific way to create a unique, unseen barrier.
There's a lot of stuff that doesn't make logical sense in the game but makes game sense. Stuff like pixel peeks for example, where the peeker has a ridiculous amount of coverage vs. the attacker. Shield/Castle is the same way. It makes game sense, as your character cannot physical reach the castle barricade to place a breach charge or melee it.
ESL can ban it all they want but it isn't an exploit until Ubisoft patches it. It's such a non-issue and in ranked on consoles, is one of the few things that actually makes Castle worth picking.