Shield Master, as a feat, is supposed to represent skill above and beyond normal shield usage - so it should rightfully require shield proficiency to take (it doesn't, as written, technically allowing you to be a master of shields but not proficient in them, which is silly), and should not grant it on its own.
I get your point. However, the original reason why most people chose the shield master feat in 5e was to be able to shove as a bonus action. Since all characters can shove as a bonus action in BG3, the feat has lost much of it’s appeal. Adding shield shield proficiency seems like a way to make it more likely to get used.
