Why do rules in different top level domains differ?
Surely you have noticed that rules for domain name registration under different top level domains differ. Why is it so?
Each top level domain (or extention) has its own operator or Registry. The Registry decides on what and how to register. For example, under .com anyone can register almost anything. Under .eu only entities registered in the European Union and citizens of the member states are allowed to register their domain names. Under .lv anyone can register a domain name but it is not allowed to register rude and offensive names.
Every extention may have additional requirements if you want to register a domain name under that particular top level domain.
By registering a domain name you accept the rules of the Registry.