Any URL shortening site that doesn't have an active block list is likely linking to some unsafe sites. Singling out bit.ly in this instance is frankly unfair.
Guess who else Google warns is linking to unsafe sites:
Although for goo.gl "6292349 pages we tested on the site over the past 90 days, 826 page(s) resulted in malicious software" and for bit.ly "91856 pages we tested on the site over the past 90 days, 721 page(s) resulted in malicious software". The % of pages on goo.gl with malicious software is much lower.
Someone with a Google Plus account embedded content (likely a block of JavaScript, iframe, or Flash object) from a bit.ly URL. That content was in turn deemed malicious.
Guess who else Google warns is linking to unsafe sites:
https://www.google.com/safebrowsing/diagnostic?site=google.c...