Why have they done this? I’m sure they have numerous reasons, but likely it comes down to one main thing – as nice a gesture as it might seem, Twitter simply does not work if you follow everybody back.
If you need specifics, think about this – your direct message inbox on Twitter is bad enough when you’re following just a dozen spammers or auto-DMers. Can you imagine what it must be like when you’re following hundreds of thousands? Even if we take a trip to la-la land and assume that all of those people are legitimate, they’re still going to bombard you with DMs, and when you get to those kinds of numbers it must be so overwhelming that one of the few legitimate options you have is to just ignore the darned thing.
via twittercism.com