Getting massive domain name informaion by WHOIS is difficult

Some companies provide massive WHOIS lookup and ask you to pay.

The difficulty is if you want to write your code to get massive Domain Name information, your IP will be blocked for a while and can only get it very slowly.

What would be the reason there is no open data for it? Some say it's regarding the privacy issue (tel, address). But is it very confidential? Those info we can get easily from search engine.

I also found some say the information WHOIS retrieved might not be 100% correct.

Is there some articles describing the above situation? I already checked http://ift.tt/1C7yYaw http://ift.tt/1c1ANj4