The investors that have pre-IPO stock and who are looking to sell as quickly as possible, gain from IPO pops, so it's in their best interest to maximize short-term profits. That group of investors include a lot of the existing investors, and in almost all cases the founders and C-suite of the company.
A large portion of those might be for checking reports etc, since reddit at least does seem to do a slightly better job trying to keep humans in the loop there.
I wouldn't be surprised if this move actually damages Reddit enough that they can't IPO because the user engagement numbers will take such a sharp negative turn.
And then it's perfectly legal.
And they continue to do that, go read any prospectus, it's all about how everything will die and the company is dead.