On one hand, they don't want to piss off all their employees with big brother culture.

To some extent, it's less offensive to mock your own stuff the bigger you are. In my limited big corp experience, everything was so big and convoluted it was never anyone's fault (150 teams worked on <thing>) something sucked and there were always 25 more teams working to fix it anyway. In some ways, it was actually helpful because people would stay engaged trying to improve things "on a big scale"

Suppressing systematic issues makes it easy to carve out a niche pretending to look useful while doing nothing (which is ultimately a waste of money)--big corp already have plenty of those

It's a fool's errand to try to prevent criticism to that extent. It's mostly harmless and functions as a release valve for people's frustrations and helps to stop them from doing something more extreme than just complaining.

It's also possibly illegal to stop them. Employees in the US have a legal right to talk to each other about their working conditions and employers are not allowed to stop them. Most companies aren't above violating that law, but they want to save that for actual union-busting, not to stop people from sharing memes.

As an individual contributor I sell my time, not my soul.

Even before the internet, employees were making these kinds of criticisms at lunch, at the water cooler and during their car pools.

If the company forbids it on visible internal channels, it will just pop up on external, private channels. With less corporate control and less corporate visibility and more leaks.