in whois.sc its paid service
webhosting.info i doubt they have.
Not sure if its legal or not todo so but its publicly available information by looking up all www.*.com and www.*.netwww.*.org and filtering your server IP to know which domains poiting to that IP and listing them.
Might want a lawyer to review zone file access agreement and see if they are allowed todo and view it to public or not.
And yeah you can always fix that by having each customer on dedicated IP so they cant figure which is which
I don't think it is the technology that is preventing them, if I remember correctly, I read somewhere at least webhosting.info is trying to make other TLDs to appear. All these are public knowledge to those who really want to check and take the troubles to check, whois.sc and webhosting.info just make life easier only.
Instead of worrying about spamming from hostile hosting co , concentrate on establishing rapport among current clients. What will be will be, let them spam, if you have great support and relationship, those spamming will have no effect.