Im not misunderstanding anything. You are trying to use wording to cheat. The action economy is a reaction, you have to choose the reaction before you roll the dice. I don't care what the flavor text says, because that's all it is. The action economy says one reaction per round. If you are DMing, and you want you players to abuse this, fine run your games as you see fit. But otherwise, you can't do both, you have to choose before the dice roll.
Come on man, nobody is "cheating" because they disagree with your view. They're using the text as written. They happen to be correct, but even if you disagree they're not cheating because they disagree with your view. Nobody is trying to use two reactions.
You allowed to choose whatever you want to do until you roll dice. Once you decide to roll dice you're committed. If the shield master ability didn't have an action economy, sure you could do both
NOBODY IS TRYING TO DO BOTH. You're not understanding what they're saying. Read what they're saying once again. You're genuinely not understanding. Digging in will make it worse - just think it through. And yes, your last response tells everyone here you're not following the conversation. You think they're trying to do something they are definitely not trying to do.