Eren has a point. But what he doesn't understand is that the Stationary Troops are a symbol. That's what Hanneth is explaining (in a very, very roundabout and actually misleading way).
When you see someone goofing off on duty, what's your first thought? They're irresponsible. But later, after a bit of time to process? You'll probably find yourself wondering "well, if they have time to goof off, then maybe they're not working enough." That's exactly what we have going on, here. The Stationary Troops aren't working enough, because they don't have to. Their job consists of guarding and reinforcing the walls. And they reinforce the walls. But guarding? What guarding? The walls have been impregnable for the past century, for god's sake. What the hell are they guarding against? Those puny Titans down there, who can't get past the wall?
Here's what the Stationary Troops are used to seeing:
The Stationary Troops are so used to this sight, they've come to dismiss Titans altogether. "The Wall's so great, and these Titans are so puny. There's nothing to worry about at all. And even if they were something to worry about, it's not like they'll get past the Wall." That's the mentality of the Stationary Troops. And it's so wrong. But they've seen this image so many times--the image of Titans, perhaps banging on the walls, maybe even trying to climb, but failing--that they've become almost desensitized to it.
So, what do the Stationary Troops really have to do as a job? Protect the people from those things? Those puny things? Pssh. That's nothing. That's not a job. Reinforcing the wall? Sure. That's work. Guarding against Titans?
There's nothing to guard against.
"Quit calling yourselves the Stationary Troops, then. Wall-building squad would be more like it!!"
And Hanneth just agrees. Why? Because that's exactly what they are at this point, and everyone knows it. They just reinforce the walls. The protecting and guarding? That's in name only. There are no troops, because there's no fighting. There are just construction workers.
Hanneth goes on to explain (poorly) that the Stationary Troops are a symbol. If the Army's active, "it means things have gone south." I'll explain in a bit how this is slightly misleading. Next, Hanneth says that "so long as [the Stationary Troops are] derided as useless freeloaders, life is peaceful for everyone." And we get what he's saying. It's true--when the Army's actively fighting Titans, it can only mean that the Titans have become a threat. And the only way they'd become a threat is by breaching the walls.
But the way Hanneth explains this is misleading. I don't know if it's a translation thing, but Hanneth explains this backwards (which I find very in-line with his character, at this point, as he and the other soldiers try very hard to only think of the Titans as an afterthought). Hanneth says that an active army means the titans have attacked. But this is presented anachronically. Titans have to attack before things can go south. And things have to be peaceful before you can be derided as useless freeloaders. This is a bit misleading for young Eren and Mikasa. Why? Because this presentation of events implies (incorrectly) that it's a good thing for the men to be drunk. That, when the soldiers are drunk, it means everything is all fine and dandy.
This is an excuse, Hanneth. Another excuse. What you're telling the kids here is "When you see us drunk, it means everything is good. Everything's fine." That is not the case! That is not the case at all! For God's sake, you will see these drunken men attempting to protect Shiganshina, and utterly failing. They fail so badly that not only do they allow Shiganshina to fall, but also Wall Maria. Wall fucking Maria.
That's what happens when you drink irresponsibly, guys. Lesson learned? Don't drink if you have people's lives in your hands.
