In a slightly less shitty world, they were planted by culture jammers.

They live entirely in their own world where all of this is completely naturalized and there's nothing but irony in the "lol fuck mondays" type of sterilized complaining about the world you can do while maintaining social niceties at work.

Remember Cato the Elder treatise about agriculture especially the part about slaves? Those are those type of people, they just cannot literally whip you anymore so they pass the time inventing new ways of psychical abuse.

Maybe they want to fire people, but don't want to pay severance?

If you put real valid complaints into sleek advertising and posters, it normalizes them. It says, "You being forced to wear uncomfortable clothes, and spend most of your one precious life around people you do not love, is a normal human experience. Just as normal and inoffensive as all the other advertising and relatable memes you see every day." In the same way you ignore the ads, and make mundane and ordinary the things you hear over and over again, they say: Ignore your discomfort. Do not let it sit unintegrated in your mind for so long that it drives you to reflection, or god forbid, to action. This is how you should react to these feelings. This is how every one of your coworkers, putting up these signs without protest, placidly walking through these doors after you, has reacted.