Not being able to sell shares in private companies is a standard practice. There are good reasons for that. The reason public market is heavily regulated is to prevent fraud.
The fact that large shareholders can sell shares in companies is also a standard practice. You might not like it but it's not capricious either. The founders created the company, the investors put money in it. They get better kind of shares than an employee because their contribution is materially different.
It's smart. That doesn't mean it shouldn't be criticized. It's forcing employees and others to take risks (specifically an equity position that cannot be partially sold to hedge against company failure) that the founders are not willing to take themselves.
If you'rea a founder in a world where you can fundraise enormous sums from private markets, why deal with the headache of going public?
Employees are correct to pressure their employers to IPO or GTFO, especially considering that many of these companies are on a trajectory where they're worth less over time (e.g. the collapsing valuations of Uber, Lyft, Slack, "We", etc).