Why wouldn't they? Otherwise, they might wait too long and the sequel would come out a few months later than it originally would have, and all the momentum would be gone! This news is actually buried in a Wrap article about Will Smith and David Ayer's other possible project together, Bright, which is apparently a cop thriller with "supernatural elements". The Wrap reports that Smith and Joel Edgerton were attached to star, and that "Ayer hopes to fit the project into his fall schedule before Warner Bros. brings him and Smith back for a planned 'Suicide Squad' sequel in 2017".
It's not surprising that sequels are in pre-production before the original movie's even out anymore. Franchises are pre-planned years into the future, and its not like planning for something has any negative consequences. Scrapping the sequel because the original completely failed at the box office is somehow less bad than having a surprise hit and then rushing a sequel into production.
Of course, when a studio has a whole sequel planned and then has to announce it's not happening, they look like they can't read the market at all. But, given the positive reaction to the Suicide Squad footage so far, Suicide Squad 2 is probably a safe bet.
This is all unofficial right now, but there will probably be an official announcement right after Suicide Squad comes out on 5 August 2016. As soon as Warner Bros. has seen it make money.