The point of the article is that Google's previously "inoffensive" spam prevention technology is being coaxed into a military tool. And this is evil for several reasons.
One: A lot of people disagree with the current US military missions, and would absolutely not collaborate with them if they were asked to.
Two: Google is a massive corporation with few or no competitors. They already have overreaching powers over several aspects of society (they're not the only one), and giving them reach over the military can only spell danger, doesn't matter how you look at it.
Three: Google is already exploiting their users in several other ways, at no benefit to them. No, convenience is not a benefit, whoever convinced people that this is the case is a genius marketer. I stopped using gmail five years ago and here I am.
The general point is: Do we really want to give ourselves always to Google, Amazon, Facebook… like that? Just because in exchange we take 5 seconds to find something on the internet instead of 1 minute?