Crostini is an official feature built by Google that allows you to run Linux on a tightly integrated hypervisor inside Chrome OS. You keep a lot of Chrome OS’ security benefits while getting a Linux machine to play with.
That said, no, it’s not illegal to install a different operating system on your Chromebook hardware. They are just PCs, under the hood. You might lose some hardware security features though, e.g. the capabilities provided by integration of the Titan silicon.
If you had a job at Google, corporate IT would definitely not be happy if you wiped the company-managed OS and installed an unmanaged Linux distro :)