Twitter is there for authentication so you can't 'vote hack' as easily - it was also much easier to integrate than a full user account system/email verificatoin.
Also, somebody else mentioned this (in a less abrasive way) - what is it about the Twitter auth that's so creepy and invasive?
Also, I fail to see why this would need to be able to read my tweets or those who I follow, assuming I even am a Twitter user.
I suppose it's not much worse than relying on Facebook or Google to authenticate, but it's not like it wouldn't be hard to create a bunch of Twitter accounts to vote hack with. Plus, let's be honest here: it's a site where you vote on funny moving pictures. This isn't the bloody U.S. Presidential Election; if you're worried about vote hacking at this stage, you're probably overthinking things.
Just my two cents/pence/$FRACTIONAL_CURRENCY.