Those website uses Twitter API and automated bots to scan, search, favorite, retweet... I remember tons of them did this and all disappeared because of policy violation (Twitter ToS). But those three seem to be around the longest. Are they NOT violating anything? I am just curious.
AFAIK - Tweepi, and others don't allow one to bulk-follow. One has to click follow buttons individually. Twitter contacted all these companies asking for this. Otherwise there would be wide-scale abuse.