You're right, not so obvious.

I wonder if the guarantee of a hostile takeover is the issue. When companies IPO, they don't expect immediate and systematic attempts of hostile takeover funded by your own money (tax dollars) through the most powerful organization in the area.

I wonder if the companies could have pulled a Zuck and kept full control regardless of ownership.

But you're right, still not so clear why they would run for the exits, unless the tax to do this was prohibitively high.