Whenever a domain name is registered, the registrar of record is required by ICANN to publish valid contact details associated with the domain. These contact details are publicly available in a "whois" index.
Whenever an individual or business registers a domain name, they agree to publish these contact details in a publicly search-able index per ICANN regulations. Who.is collects this public information and adds it to existing archives. Every addition increases the knowledge and resources available to individuals using Who.is. Much like a library that collects newspapers containing publicly available information, Who.is maintains a large amount of these records to aggregate and expand the collective knowledge available online. This knowledge can be used in a variety of ways, including academic pursuits, business research and general curiosity.
Removing this information does require a nominal fee. This fee not only reflects the cost of actually implementing the removal, but also reflects that visitors to Who.is will no longer have access to once publicly available information. The total amount of publicly available information through Who.is will be lessened.