"Punishing" players for being naughty and playing the game effectively is kinda like admitting that we suck.
Incentivising playing the game (aka the carrot) inevitably leads to more players
enjoying the game and it should not be overlooked that this is the whole dang point of a game.
Adjusting loot quality up/down is super easy at this point so that was simply the first thing that came to mind.
Killing those horde zombies could also
lower your gamestage while leaving the loot quality the same. So if you go out looting afterwards, you'll have an easier time because you "thinned out the zombie population". How cool is that?
Right now those hordes are a complete result of what you did the rest of the week. But what you do the rest of the week could also be affected by whether or not you were killing all those zombies...
A lot of these approaches were simply not feasible pre-A17 (without lots of hacky workarounds) because the game data was littered over so many places. That "useless" overhaul and consolidation of game systems is what
allows us to do things like the above in a matter of hours.