> Also I am interested what are some open source projects which became closed source...

The most prominent one is Solaris. It was open one day, and closed the next. Memo didn't say we're close-sourcing it, but moving to a cathedral (final release as open source only) model, but they never released the sources ever after.

Oracle lost all of the core developers over it.

This where Illumos took over.

> There must be some legal laws protecting it.

For permissively licensed code? Nope. Nothing. Even if you don't transfer the copyright, nothing stops someone from forking it and building on it closed source. That someone would be the company opening it or someone else.

In the olden days, when the internet was not that capable to allow collaborative software development, losing developers was a real threat. Now it's not. Developers are dime a dozen. You can close the source, hire some people and continue working on it.

However, this is Open Source model working as intended. Freedom to the developers! If a developer wants to work on a closed source fork, it's completely permitted, baked into the system.

This is why GPL (esp. v3), while viral, is superior. You can't change the license if there's a copyright holder other than you. You can't just fork and close the source. It's limiting to keep the freedom. A working (and arguably necessary) compromise.