It’s completely valid for them to do so. Their Top Level Domain meant for their purposes.
Even though it’s rarely enforced, many TLDs have specific purposes and conditions against abusing it for unrelated purposes.
There are many others to pick from.
The fact that people were registering .ml domains for projects like this is mindboggling. There are many TLDs to pick from without infringing on the terms of use of a country-specific one.