Re: Why don't they...
Corps shouldn't be allowed to register TLD. It's too short-sighted. Who's to say Amazon won't be a husk of a company within a decade?
That may be so, but if Amazon goes bankrupt or finds other financial incentive (e.g. planet full of megaton gold dubloons), Amazon can sell off the TLDs to another corp (Yahoo, for example).
Nations are not immune either - the .USSR or .sikkim domains are not in high demand right now.