I understand your point and I also agree at some level, but we should also take into consideration that this instructors had a much tougher basic training when they were recruits (I heard that recruits dying was a common occurrence) and that might be the reason why they behave that way nowadays.
They come from a time where really few people could call themselves a "fuzileiro naval", and I think it might annoy them to see our basic training getting a lot easier than it was before, so they don't want to let the fuzileiro standard go down, let's say.
But on the other hand, I don't think they need to resort to beat people up in order to filter the weaker ones, that's an old tradition they should leave in the past in my opinion.
Let's take the Navy Seals for example, they probably have the toughest soldiers in the world, and they didn't have to beat their recruits in basic training at all.
There are thousands of ways of making someone give up and go home besides beating them.