Trying to justify your position with even more controversial rulings isn't a wise thing to do. Now we are going to spend another 70+ pages arguing about those controversial rules
Hand crossbows were eventually ruled to require a hand to load them despite the rules not originally stating so. They did this because you have to be able to load the bow to fire it.
Consider this: Suppose a PC keeps his shield raised above his head and not deflecting any blows with it during a fight as a matter of honor to show that he doesn't need the shield to defeat said opponent. Does he gain the +2 Bonus to AC? I would say no. So a character can have a shield equipped and not gain the AC bonus IMO. Does attacking with the shield cause that to happen. That to me is the DM's decision. One logical decision is that attacking with the shield means you aren't defending with it and so it essentially is the same as holding it above your head.
So I don't think you've provided a good example of what you are saying.
By the way, speaking of this, if you can just bonus action with the shield constantly then why even bother with shield master? The obvious answer is that they didn't write the rules with the expectation that TWF would work with shields in any way that would provide the AC bonus, the dueling bonus and the TWF bonus.