When you buy the TLD, you're not just reserving it for your own use -- you're signing up to operate a registry which will allow anyone else to purchase domains

When you buy the TLD, you're not just reserving it for your own use -- you're signing up to operate a registry which will allow anyone else to purchase domains under your TLD. ICANN wants verification that you have sufficient funds to run a registry, for one thing. They're also a non-profit organization, and they seek the fees to support all the copyright arbitration and legal wrangling that will eventually ensue over the new TLDs.